OBTAIN A SESSION ID
Like many other API calls, the locker load call requires a session ID. See the documentation for the login call in the online documentation for more details, but basically it looks like (using XML output):
https://shop.mp3tunes.com/api/v0/login?username=<email>&password=<password>&partner_token=XXXXXXXXXX&output=xml
Grab the session ID from the result and save for future use.