So to start no matter what you need
2 DS SYSTEMS. I used my DSiXL and a DS Lite.
So you have to reach the end of the main story. So with th first DS system put in your White or black versions and start it up. ( I did this right after I beat the main story, so mainly I am just going to tell you what I did.)
Once you are back in your house go through the talking and stuff. Once all the talking is done including where you get the National DEX. USE "Fly and go to Nimbasa City. From there head to route 16 and move onto the Marvelous bridge (Gate is to the far right, just follow the road) head across the bridge, as you do this you will get stopped by the 3 ninja dudes(forget there name) They will talk with you and give you some items. anyway once across the bridge you will see a trailer to the right you should see a set of stairs. Head up and into the building.
A female scientist will talk to you. then take you up some stairs. where more talking insues and then a male lead scientist will talk with you. Anyway once all the talking is through they will go through some stuff that is like what you used to go through when entering a Union room. It will end this with a save game.
Okay now we start the fun.
So you will walk upto a giant screen. the scientist will start the poke transfer thing. On the top screen will appear an image showing a "DS Download Play" (For me it showed the DSi version. I don't know if it does the same on a DS lite or if the image will be different.) Anyway on the bottom will appear a Lot of text which will read like this.
Please Insert a Pokemon Diamond, Pearl, Platinum, HeartGold, or SoulSilver Version Game Card in the other DS system and turn it on. Then select "Poke Transfer ID: XXXXX" from "DS Download Play"
I switched out the trainer ID with X's. IDK if it is the same for everyone or different.
Anyway once this has appeared place it in a safe place and pick up the second DS. Now before I continue make sure that the shiny Raikou, Entei, or Suicune is in a pokemon box and not in your party. So once your Legendary dog is in a box. place the game into the second DS and start the ds up.
DO NOT START THE GAME Once the main DS menu has appeared select "DS Download Play" Give it a few to find the software. It should show up saying "Poke Transfer ID: XXXXX". Choose "select" and "yes" to start the download. Give it a few to Download.
Once it does on the Touch screen your pokemon boxes on the game inserted into the DS will appear. Find your Legendary Dog (I will say Entei from now on to make this easier) that you wish to transfer (I would suggest only doing 1 if you have multiple. IN truth I have 2 of each spread across 3 pokemon games.) Using your Stylus drag Entei over to the right side of the creen and into a slot like that you would have in a pokemon party with you in regular gameplay. Now you have to put 6 pokemon (just put cannon fodder or ones you just plain don't care about also you can not use ones that know HM moves) Once all 6 slots have a pokemon it will say are you sure about this. tap the screen till you get to the "yes" or "no" prompt. Mainly a final confirmtion checking you want to do this. Cause as said once you transfer them they can never go back.
Hit yes and a small game begins (may take a little don't remeber if it was immediate or not to start) You have to use a weird croosbow type thing to lauch pokeballs to captur the Pokemon you are transfering. It's pretty easy to figure out. follow on screen prompts to finish the process. It will ask you if you want to transfer more. I said no and it will back you out. It should now say that "the process was closed and you can turn off the power" or something like that. Anyway once you turn off the 2nd system move back to the 1st one with the white or black version in it. The scientist guy will talk to you again and say that he placed the captured pokemon in your PC.
Now onto how to get Zoroark. Now that you have your Entei.
So of course go to the PC to the left and place Entei in your party. Then head back to route 16. Now when you went through route 16 to get to the Poke Transfer Lab. You may have noticed some trails leading up. Well take one and go to the top-middle of the map. You should see a nice old special area type door that leads to "Lostlorn Forest". Same type of door that lead to the Dreamyard where you met Muuna with Bel/Bianca.
Now before you go in make sure you have a somewaht weak pokemon. Mainly you need to injure without killing zoroark to get it out of it's illusion it starts the battle with. Anyway once all set go into the Lostlorn Forest. Again head for the top of the map. After a little you should see a trailer. Head towards it like you are going to go in. ONce you reach a certain distance a person will come out and charge you then makes a Pokemon cry. A special battle start will happen and you will enter the batlle against Zoroark. Now as said You need to get Zoroark out of it's illusion. It will start the battle by appearing as one of the other Legendary dogs. hit it without killing it doing so should cause the illusion to stop working turning it into Zoroark's true form. Now you can capture it.
I used a Master Ball to capture.
Anyway hoped this help if you were having trouble with getting Zoroark or trying to figure out the Poke Transfer System.