To add an Application, click on the "Add Applications" link that appears in the left column of your profile page, beneath your photo:
This will take you to the Application Directory, where you can browse all Applications or view them by category. When you find an Application that you'd like to add, click on the "Add to My Page" button and then confirm by clicking "Add Application". The Application will now appear on your profile!
You can also add other OpenSocial Applications that do not appear in the Application Directory. To do this, click on the "Add By URL" link at the top of the Application Directory page. You can enter the address of any compatible OpenSocial Application and click the "Add Application" button. After confirming by clicking the "OK" button, the Application will be added to your profile.