Put your mouse cursor over where it shows your username, (next to Inbox) select 'Avatar', then enter your username in the bar where it asks for your Minecraft name. You can also upload a skin file if you don't want to enter your username. (It automatically pulls from your in game skin)
Hey ham! That even helped me too! Thanks even though you didn't mean to! Oh and how do you do the "upload skin thing"?
Select avatar from the drop down menu where it shows your name, click 'choose file,' then select a skin from your computer