Hey there- as far as I know, there is no ingame "unlock inactive character" option. It is likely as stated above that you unlocked an extra character slot, log out and recheck. if not, calling customer care can usually rectify your situation.
Also, if you go to your official account on the main website, and go to ledger, it will tell you exactly what you purchased!
Hope that helps,
Yespa
(This was my response, but on the wrong account.)
