In your "welcome" email from LP, you should have been given the two LP nameservers to point your domain to. The procedure for exactly how to do this varies by registrar, so you'll have to ask them how to work their control panel. The names should be something like "NS6.LUNARPAGES.COM", and you enter both entire names.
Once that's done, your domain name will resolve to your LP site. Don't forget that it can take up to 72 hours for this information to propagate through the DNS system so that when you type your domain name into your browser, it translates it to the LP nameserver IP address and on to your site.