Google makes it clear that it doesn’t support the redirection of naked domains (domain.com is a naked domain, www.domain.com is a subdomain). However, you can do it. I host some sites with Enom, so there were simple instructions to follow.
Summary: change your @ (and * if you like) host record to a URL Frame Record type and the address to, e.g., http://sites.google.com/a/domain.com/main/.