ask how to make form login to DB_Cart

I'm Kahlil, programmer from indonesia,
 
I wanna make the form login from your DB_cart class,
 
but on the check out, I still got the trouble, because the variable to give the parameter
for SQL statement ( for Id values ), couldn't work properly...

so every user that logged in will get the same value on check out form.
 
do you have the reference to me, to make the great form login to this class?
 
I'll wait for your response.
thanks.
Kahlil
http://kahlilstt.blogsome.com

I don't have an example how to show you but I used this class together with db_cart several times:
http://www.finalwebsites.com/snippets.php?id=10