I've read through the game guide and FAQ and everything says "Expansions can be bought at the Barter." Actually all the guides say there's a lot of things at the Barter but nothing it mentions in the guides shows up there for me, do I need to do something? Is my account not old enough to purchase wolf slots? Or any of the other Barter items?

Go to Realm > Marketplace > Barter > Apples to find the option to purchase a Den Expansion. Each expansion costs 10 apples and adds 5 permanent wolf slots to your den.

Realm > Marketplace > Barter > Apples. You can buy a den expansion for 10 apples and get 5 extra permanent wolf slots.

Ooh I see, thank you ^-^ Is that the only way to be able to have more wolves in your pack?

If you are premium and specialize in breeding, you can get a maximum of 3 extra den slots