MyBB Community Forums

I'm looking for how to put absence ads on my forum, all the plugins I see are not compatible with my version.

Absense ads? I take it you mean AdSense, in which case I highly recommend MyAdvertisements:

http://mods.mybb.com/view/my-advertisements

How do I use it?

Did you even try to use it? It should be pretty straight forward.

Yes, but how do I place ads and choose where?

You have clearly not even looked. There's a sub-section under Tools & Maintenance which allows you to easily manage them.