hi there -
I was installing qbms.php on a shared host, and after going through the whole certificate/callback process realized that I had to use "Desktop" since the IP number which QBMS sees from my host is dynamic and does not match that of the website's IP. "Desktop" is also a lot easier to configure overall as theres no certificate/callback madness.
I adapted the XML snip posted on the Intuit support thread into qbms.php and added options for "Hosted/Desktop" and "App ID". With this version, the certificate file and originating IP are only needed if you selected "Hosted". The new version is attached to http://addons.oscommerce.com/info/5383 .