In the help menu it says that a post will have an Add to Pool option that let's you add it to a pool. I have been looking very carefully at the options presented with each post but I can't see a Add to Pool option. Is it something that onyl appears when you yourself create a pool? Is it that you can only add image to a pool you yourself created?

You need to turn on Advanced Editing Features in your account settings. That'll fix your problem. It also looks like some of the Danbooru pool features referenced in the help files are incomplete/not implemented in this software, if you're looking for anything else.

Yeah, I had this problem when I was trying to set up the site. We did ask the developer if there was any way to make the Advanced Editing Features on by default.

Oh, wow, I would ask something about pools too...
About it:
"If you're importing several posts into a pool, this process can become tedious. You can instead click on the Import link at the bottom of the pool's page. [...]"
Where? I don't saw that... And I have checked Advanced Editing option ^^

That option doesn't exist, at least not yet. Like petal said, there are some things mentioned in the help files that are incomplete or not implemented yet.

oh, understood :)