If your login name does not authenticate into PhotoPost then one of two things is wrong in your setup.
1. Check your config-inc.php file and make sure the $vblicense variable is the license number not customer number. You can find your license number on the top of most vb
2. You will need to use phpmyadmin for this one but check your photopost settings table and entry 172 should be set to your vbulletin cookie prefix which by default is bb. You can double check the true prefix by viewing the vb
config file but your prefix needs to be set in Photopost to match VB
Now the reason these are so important is that the password for the user for the cookie is encoded with the vb
license number and cookie prefix so if you do not set this correctly the login is invalid.