I have seen a number of posts of people having difficulties with adding their Minecraft character to their profile page. Hopefully, these instructions will be useful for you to add your character:
To add your Minecraft character, follow these steps:
(1) Click your avatar to go to your profile page
(2) Click the Characters tab
(3) Add Character
(4) Select Minecraft
(5) You will now be asked to enter a code. Do the following:
--(a) Start Minecraft on version 1.7.2
--(b) Click Multiplayer
--(c) Click Direct Connection
--(d) Enter: mclogin.enjin.com and then click Join Server (see image below)
--(e) You should get kicked and a code shown
--(f) Enter that code into your Add character
--(g) Your character should now be verified and added
Please note that, after you click Multiplayer, you click Direct Connection in order to enter the mclogin.enjin.com.
Video on how to do this:
I hope this has been helpful!
EDIT: You MUST have a premium Minecraft account for this. Cracked accounts will not work.
UPDATE: I have been informed that you may need to set your Launcher to Version 1.7.9 to do this.