It tells me I need to press a button...

I press Mouse 3 and get a spellbook. I click the Destruction spell. Then it says something like "Press a button".
Pressing Mouse 2 says "You've already got a spell loaded".

WHAT DO I DO!?@#!

On touch? Distance? Area effect? It's just the 'activate' button, usually mouse1 or spacebar or something like that. But for on touch spell you have to be in contact with the enemy.