Go to "Edit Profile" and then scroll down to were it says "Select Avatar." In one of the boxes it says "http://" and in that box you put the link to the avatar that you want. Then you can either press "Preview" to see if it works or you can just scroll all the way down and press "Update Profile."
Also, if your avatar doesn't work, check the link to see if it's correct and if it is but the link still doesn't work, check to see if it is no bigger than 90 x 90 pixals.
I hope that helps. If not, I'll put up a visual if you want, but later. ^^
<font color=PINK><center>Believe in Yourself</center></font>