How do I activate it? Here's a picture of what I'm talking about. http://i.snag.gy/ODKx1.jpg
That spell overrides your current right-click spell by using TAB (default bindings).
You unfortunately will need to TAB between those two spells to use them.
If that proves too slow for you, you'll have to bind it to another key. Notice you already have it bound to 5.
The answer to this question covers the keybinding issue.
You can also click to the spell menu (not the spell tree) and aim with your mouse to the spell that you want to key-assign, then press one of the function buttons (by those I mean any of them: F1, F2, F3, F4, F5, and so on...).
So when you want to use (e.g.) you assign a fireball spell, which was assigned to F1, you press that and there you have it.