I have the same problem when trying to install moodle. I just set up my web site to be hosted on powweb.com. Initially I attributed the error to some technical problems powweb is experiencing but your similar problem points to something else. I am also using a Mac with OS X. I tried four different web browsers (Safari,
IE,
Firefox, Mozilla) and they all get the same error. In fact, I'm having trouble just starting install.php. I get the following error...
"Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, xxxx@xxxxxx.xxx 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."
Good luck,
Germán