From their API site:
What is the API user access key?
The API user access key is a unique alphanumeric value (string) that, together with the user name and password, is used to validate any calls to the API. This unique key is issued to individual users at signup and provides more security (authentication) than only a username and password.
How do I receive a user access key?
Contact your local sales representative. Your sales representative will assign a search marketing specialist to you who will give you a user access key.
Can I have more than one user access key?
Contact your sales representative to have more than one user access key.
What do I do if I lose my user access key?
Contact your sales representative to resend you the information.
oh well thank you, i guess this is how you keep your useless sales reps busy, Microsoft.