Hi xtech
I now this isn't quite the answer your looking for I also tried to do this but couldn't figure it out, so in stead I brought System Messages mod for X-Cart from Steve at cflsystems.
So much more professional looking and does a lot more.
Its not expensive either won't break the bank.
http://www.cflsystems.com/system-messages-mod-for-x-cart.html
Just a thought if you don't get your problem sorted out...