Well, it looks like you have a bit of a mess going on. It's really hard to tell where you might have gone wrong without access to your error logs (that's where you will get your best clue(s) as to the nature of the problem), and a generic "Internal Server Error" doesn't really tell you much.
Also, you wrote:
This is strange, as I have no idea why you would go to your "email" to login , and have no idea what you are really trying to say here.
From looking at your site, it looks as though you have attempted to install your own PHP, and have that pretty well "borked" - http://businesslinkd.com/cgi-system/ shows a strange configuration (and I'm not sure why you would even need that. You can run WordPressMU with the installed DH PHP (no need to use your own).
Finally, browsing to your site at http://businesslinkd.com/ and entering the WPMU directory http://businesslinkd.com/wordpress-mu-1.2.1/ produces additional error messages that indicate either the software is not installed or your paths are hopelessly borked.
You need to "undo" whatever you did (except the apparently functioning Joomla! installation), and start over. WordPressMU is notoriously "difficult" for beginners to install, and you cannot use the wildcard dns functionality to house the users' blogs without special request to DH support to have this enabled for your domain. This is not suitable for "beginners" .
At any rate, if you intend to install WordPress MU, I suggest you undo your earlier attempt, and carefully follow this DH Wiki article on installing WPMU. Note also the link at the bottom of the page to a different tutorial on installing WPMU on Dreamhost.
You might also consider an alternative, which is much more easily installable on DH, which is Lyceum, a multi-user/multi-blog fork of original WordPress code. Installing Lyceum should take about 10 minutes if you read the documentation carefully.