Please make sure you set "default" templte for your X-Cart shop at X-Payments admin back-end - see
http://help.x-cart.com/index.php?title=X-Payments:User_manual#Adding_an_Online_Store
If you want to use "iFrame" integration you need to set "lite" template at X-Payments side and enable "lite interface" support for X-Payments payment methods at X-Cart admin back-end side.
Also, X-Cart 4.5.5 is recommended version for using with X-Payments 1.0.6.