Rotom is obtained the same way as in BW1, and his forms are changed the same way too. Trade Rotom for a Ditto with a lady in a trailer on Route 15. It will be level 60 with a Gentle Nature and holds a Max Elixir. Change it's form by going to Shopping Mall Nine, going into the storage room at the back on the right side of the store, and once you are in interact with one of the boxes. From their it will give you a list of appliances Rotom can enter, each one making him transform into a different form.