It is just like the normal furnaces, except that it interacts better with pipes and adjacent inventories, and that it needs to be supplied with Buildcraft energy instead of feeding it coal.
Build a stirling engine and place it next to the powered furnace so that it faces it (will happen automatically when you place the furnace first, then the engine). Then put coal into the engine and place a lever next to it to activate it. Your powered furnace now has energy and can smelt stuff. Enjoy!
EDIT: Also, this might help you:
http://forum.feed-the-beast.com/threads/getting-started-with-feed-the-beast.4134/