Where are you when you're trying to access your server's remote access web site? If you're at home, could you try from a remote location?
Also, make sure that you've properly configured your SonicWall router to forward the ports. While the manual you received with the router is adequate, I think
this page is more useful in this particular case, as it leads you step by step through configuring a port.
I'm not on the WHS team, I just post a lot. :)