Use the username and password that you chose when setting up your AMA St Louis account. A copy of them should have been emailed to you upon opening your account (or if an admin setup your account for you). Check your spam box if you haven't received it.
If you have forgotten your password, click on the 'Forgot Your Password?' link to the left. Enter your username, and you new password will be emailed to you.
Once logged in, you can change your password by clicking on the 'Account' link at the very top of the page and then on 'edit' above your information.