You could use an if/then statement in head.tpl
http://forum.x-cart.com/showthread.php?t=8881
Another useful place for this tag is on your static pages - many people have complained of not having "content protection" on x-cart - well, this is how you do it. Create your static page, make sure you have "parse smarty tags in static pages" checked in your general setting for static pages, and then at the top of your html where your content begins, simply put that tag in there surrounding your "real" content, and then an {else} for your message to people who don't have the membership.
Something like this:
{if $user_membership ne "Level 1"}
I'm sorry, you don't have a membership, so this content is not accessible. Please click here to become a member and get the goodies.
{else}
Your real html content
{/if}