A very noonish question, but....How do i activate it? 

Nothing to activate. It’s already there. You just need to put something in it. 

Look on the left side of your screen. Do you see a small tab with an arrow on it? Select that and you can then drag and drop from the available options.

On pages I have seen the little tab to add blocks, but I can't say I have seen it on the forums.

33 minutes ago, Kurisu87 said:

On pages I have seen the little tab to add blocks, but I can't say I have seen it on the forums.

It should be available on every part of the front-end of your site (certainly throughout forums) and appears on the edge of your window, like this:

Found it, thank you. You know, I never noticed it on the forums lol