Simple problem for experienced PHP user

Hi,
I hope someone can help me.

I have just installed a PHP membership script on to my site, now the problem is, when I try to log in with the administrator credentials, which were provided with the script, it says invalid log in.
Apparantly I can edit the credentials for the admin user and pass in the database, but I'm unsure how, I've found the fields that I need to edit, but what do I do?
I've managed to register another account using the scipt, but I can't do anything with it as it is asking for payment, as I haven't been able to disable the payment feature with the admin account, all a bit of a mess really.

Also, I can't seem to log in to any account, even the one I created, I can't show you as I cannot post URL'S, but it says invalid login credentials, even though I received the email with the login credentials that I entered.

Not sure whats wrong, at all.

The fields in the database read:
admin_user
admin_pass

both of those are varchar (255) fields, not null and collation of latin1_swedish_ci

There is no other information in there.
I am unsure where to enter the admin credentials that I need to use.

I hope someone can help, I've been trying for days now.

Thanks for reading my post.

I would try contacting the developer of the software, they usually have support forums. It could well be a bug that needs to be fixed, other than that i would suggest you backup the current database in phpmyadmin (or something similar) then just "reinstall" it, it may work afterwards.