Check the AA window (V key by default, or Inventory window, Alt Adv button). I forget which tab, but believe it's Special tab. It's free at the appropriate level (14? it's 6, if I remember correctly). Select and press the Make Hotkey button, then place it in a hot bar.

EDIT: Or make your own hotkey with the command /alt activate 6000.

Yther Ore.

Edited, Oct 2nd 2011 5:35am by Yther

Perhaps things have changed again. As of right now with my SK this is available at least by level 6. (I am pretty sure it is level 1 ability). You can hotkey it by going to your Alt Adv button in your inventory, select the Class tab and look for Harm Touch. Click that and then press the button that says Hotkey, it will appear on your cursor and you can place it on your hotbar.