At the top of all reply and quick reply you have a button that says LINK.

The first line is the URL and the 2nd line is where you want to link to. So [link=http://www.rcuniverse.com]RC Universe Main Page[/link]

Would be
http:// (the default) then www.rcuniverse.com in the first box and RC Universe Main Page in the second.

Also anything that starts with www. or http:// will automatically become a link.