Go east from Marvelous Bridge to get to Route 15. The Pokétransfer Lab is here.
Get a Rotom
If you give a Ditto to the woman in the RV, she will give you a Rotom. You can find Ditto in Giant Chasm.
Use the Pokétransfer Lab
If you have two DS systems, you can use the Pokétransfer Lab. Put your copy of Diamond, Pearl, Platinum, HeartGold, or SoulSilver into the other DS. If you transfer a pokémon from one of those games to your copy of Black and White, you can never send it back, so be careful! You will have to remove any items from the pokémon that you are going to transfer, and take it to the move deleter to remove any HM moves that it knows.
Get TM09 Venoshock
You will find a boulder near the Pokémon Ranger. Push it some of the way west, then push it north and then east into the hole. Beyond it is TM09 Venoshock.
Get the Up-Grade
To the east is another boulder that you can push with Strength. Push it into the hole to find the Up-Grade. If you give that item to a Porygon, it will become Porygon2 when traded.