I've had Platinum for about a month and a half, and Ibai posted all the different ranks for the forums a while ago, and it includes the donor ranks. I wanted to know when everybody's ranks will be added to their forum profile.
Currently the only ways for the administration to assign forum ranks is by doing it manually. This means that if you want your forums rank, you will need to message Ibai, providing him your forums username so that he can assign it to you.
I hope this solves your issue. I will keep this thread opened just in case you have more questions