You're in the wrong place - at the top right corner of the page, instead of 'My Profile', click 'Settings', and then on the left side under the heading 'My Settings', click 'Edit Character' and it should show at the top your currently set character and below that, a list of all your characters on your account.

To select a desired character from this list, click on the grey star beside the character's avatar picture, and the star should turn yellow. Then select the three 'main' classes/Jobs for your character (based on their actual level in game) - the left most one is the one that will appear under 'Main Class' on every post you make, and will determine your posting restrictions, so it's best to select a class or Job with the highest level (at least 50). Then when you're satisfied, click 'Save Changes' at the bottom right corner of the page.

Hopefully this has helped.