Because you have less then 10 posts in the forums..
To be allowed to make an ad, you need to have 10 forum posts, and be a member for 30 days. You already have the 30 days thing done, so after 5 more posts you'll be able to. And just so you don't make 5 random posts of nothingness, after you reach the "allowed to" level, it'll still take about a day for the place an ad button to appear.
Cosplay DA Regular DA
Kumoricon 2014 (so far):
-Sora (Kingdom Hearts 2)
-Alice (Hearts no Kuni no Alice)
-Black&Blue Loli-style Dark Magical Girl O.C.
-Akaito (Vocaloid. Black & Red Ver.)
Dream cosplays for now:
-Tsuruya-san (Haruhi Suzumiya)
-Kagami Hiiragi (Lucky Star)