I just don't get it, how does one cast a spell? I know that I select one using the "cast spell" key, but I just can't find the "fire spell" key, can someone help me?
If you mean when it says "Press key (or something like that) to fire spell", you have to re-enable the mouse cursor and click on the screen. By default I think the ENTER key does this.
If you're using the cursor, just clicking on the enemy will loose the spell. If you're using mouse-look, then press the spacebar. Remember that you actually have to be in range to touch someone if you use a touch spell.