How Do I Use Reactor?

[Arlayn]

I have been working on my Ash warframe and have finally unlocked all four abilites and found I cannot use the fourth. Then I learned I needed a reactor so I got the reactor, but there is no button or indication of how to use the reactor. Am I supposed to max out the warframe or am I missing something?

[Liacu]

Go into your warframe's skill tree and there should be a picture of the reactor on the top left, underneath the warframe's skill point count.