Once properly logged into the forums (with your Battle.net email address and password), at the top right corner you should see a character listed -- this could be for World of Warcraft or StarCraft II. Just above that character's name should be your BattleTag nickname. Next to your character's name you should see a down-arrow. Click on that to display your available character list. This list will feature characters from any of the games you have associated with your Battle.net account.
If you are not already set to your Diablo III BattleTag, go to the bottom of the list where you should see an option to Manage Characters. Clicking on that will expand the list of your in-game characters and accounts. At the very top (for most folks) you should see the Battle.net icon, the Diablo III icon, and your BattleTag name with the associated hash and unique identifier numbers. Select that and you're all set! The View Profile link should now reside right underneath your name where you can check out your characters and their shiny gear.