Just to kind of sum up how this is obtained:
At level 51+ you have the option to divert experience from your normal experience bar for leveling into AA experience (Hit 'V' in game to see the AA screen). Accumulating points will allow you to purchase special abilities, such as stat increases, innate powers (run speed, enduring breath, etc) and spell type abilites.
Purify Soul is one of these spell type abilities. Once you have spent enough AA points to meet the pre-requisites you can then purchase this ability (which will be listed in the AA screen when you click on the desired ability). To use the ability you will go into the AA screen again, click on the ability name, and on the right side choose "Make Hotkey". Drop the hotkey in the hotkey window with your other spells/etc and you're gtg.