osCommerce site - fix checkout process with credit card

We have 2 websites: [login to view URL] [login to view URL] Both sites works under osCommerce platform, and was build the same way, just different look. The shopping cart works with PayPal and credit cards ([login to view URL] Credit Card AIM). The site [login to view URL] works good with PayPal and credit cards. The problem is with the site http://www.nailart4less.com. It works good with PayPal, but in the credit card option your'e getting the error message "There has been an error processing your credit card". We need to fix this, so credit card purchase will go through successfully. The programmer will have access to both sites admin, and to the files. Also to [login to view URL] site if needed. Thanks Eden Katz