I am installing Moodle locally on a Mac. When I go to run install.php I get error 500 message.

Try changing the following lines in httpd.conf: 
 AllowOverride All 
 To: 
 AllowOverride None 
 rename .htaccess 
 Actually, I think its an .htaccess issue. Subversion passes the .htaccess file in the checkout unfortunately. If you’re just testing locally, you can rename your .htaccess file to something else like .htaccess_bak and then try again. This will ensure that you have access to the moodle directory. The .htaccess file tells apache to look for shibboleth authentication before allowing access to the moodle directory.