WordPress MU

I was intalling the WPMU2.7. I followed the guide to put all the unzipped files to the folder public_html/ and open my web at XXX.com/index.php. The following web came up:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, webmaster@xxx.com and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

--------------------------------------------------------------------------------

Apache/1.3.41 Server at xxx.com Port 80

please help me figure out what happened, thx!!
BTW, I'm using the Lunarpages host server.

Are you able to see a file named .htaccess in your install folder? If so, try renaming it to something else and load your page. This file is necessary, but it gets us closer to the problem.

The other thing to do would be to look at your HTTP error log file. Not sure how you do this on LunarPages, but that file would give you the error information we will need to provide further help with this issue.

thank you so much. I will check that,
.htaccess is the file type. If it is neccessary, how can i possibly change the file type?

thats a linux file called .htaccess don't worry about fiel types, jsut look for this file.

And what are the permission on public_html/?

Is this an add on domain to your account?

re: andrea_r

permission is 777, permission for wp_content is 755.
and I'm talking about my main domain.

And I'm just double-checking.