You sit in the mushroom circle to get the spell, but you can only give it to someone else. Someone's got to give you the spell to turn you into another animal or a minideer.

what you have to do is sit inside the circle until your deer falls asleep. You'll hear the "bling" sound like you do when you eat the tree mushrooms or a pinecone, and that tells you you have the spell.

It's tricky though because you can't move your mouse for about 5 minutes, or else the timer resets. So the best way to do it is to sit in the circle, and do something else for the 5 minutes (either surf the internet or leave your computer for 5 minutes). But remember you have to give the spell to someone else, you can't use it on yourself.