I just purchased the Flexible Content Field add-on and I do not see Custom Fields -> Settings or anywhere else to enter in the activation code I received. I see that this add-on is installed but I do not see anything that resembles this add-on when I go create Field Groups.
I recently just upgraded to WordPress 3.8.3.
There is nothing to activate – just start using it, it will work. The activation codes are a remainder from V3 if I remember correctly. It’s working fine without activation anyway.
ACF v5 will have an activation code though.