You need a standard User ID and Password in order to use Passpack Desktop with your Passpack.com account. It's OK, you can have both an OpenID and a standard login.
Login to your Passpack online account and set up a standard User ID and Password. You can do this under Settings > Set User ID and Settings > Set Password from inside your account. Use those credentials to login to your Passpack Desktop. You'll only have to do this once. After, you'll just be asked to insert your Packing Key into Passpack Desktop. You can continue to use OpenID in your online account as you always have.