How do you transform Rotom in black and white?

You can transform Rotom by going to Shopping Mall Nine on Route 9. As you enter the mall, on the bottom floor at the very right there will be a doorway which leads to a storage room with a man inside. He says, "This mountain of cardboard... They're all full of electronics we can't sell anymore. They're just going to waste. I wonder if someone could put them to good use?" Go up to any of the boxes and press A. It'll say,"It's full of cardboard boxes with electrical appliances in them. (If you have Rotom, it'll say->)Oh? Rotom would like to investigate the motors of the electrical appliances... If you select YES, then you can choose which motor you want your Rotom to enter. There is:

Microwave Oven --Rotom will become Fire Type and can learn Overheat.

Washing Machine--Rotom will become Water Type and can learn Hydro Pump.

Refrigerator------------ Rotom will become Ice Type and can learn Blizzard.

Electrical Fan----------- Rotom will become Flying Typeand can learn Air Slash.

Lawnmower------------ Rotom will become Grass Type and can learn Leaf Storm.

Rotom will turn into these objects. You can obtain Rotom on Route 15 in a trailer near the sign. The scientist inside will ask if you can trade a Ditto for her Rotom. Which means that you can only obtain Rotom if you have Ditto in your party. You can obtain Ditto at the Giant Chasm near Lacunosa Town. You can go to Giant Chasm with the HM Surf. Go down to the ocean side near the old man and surf upwards and I'm sure you'll find your way into the Giant Chasm. hope that helped.