I believe that you need to click the button on your quickbar (Activate the ability), then cast a touch spell targeting anything you can (I usually cast it targeting myself, or, if PvP settings don't allow it, an item in my inventory). You will cast the spell, but instead of having an effect it will say something along the lines of "Spell Stored".
All you need to do then is whack something with your weapon, and the spell will go off.