Hi Wilco,
You seem to have problem with permissions for user chaospag_osc1, this user must have permissions to connect to, alter and so on the phplist database.
This mod is made in the way that is uses the same database user for both databases. Ie. it uses the username that has privileges for the Oscommerce database to connect to the phplist db, because of this you must set up the same db user for both databases or give the oscommerce db user privileges/permissions to the phplist db.
This can be done through commandline, phpmyadmin (if permitted by hosting provider) or by some other way supported by the hosting provider.
If you can't control creation, permissions or other handling of database users, I am sorry, this mod is not for you (at least not in its present state). I don't have either the need for or knowledge in rewriting the mod for this purpose.
Regards
Thomas A.