Problems setting-up multiple databases for Multi-db plugin

I've been following the usage instructions for Multi-DB, but I'm stuck at this point:

"One final step is to make sure you have assigned a username and password to be associated with each database."

How can I add a user to multiple databases? And how can I remove the "admin" user from these databases?

I have a VPS from 123-reg using Plesk Panel 11.0.9, phpmyadmin and Webadmin