Installation Warning: mod_rewrite problem

I successfully installed Omeka!

But, now this:

Installation Warning

The following issues will not affect installation, but they may negatively affect the behavior of Omeka:

1.
Unable to check for mod_rewrite

Unable to verify that mod_rewrite is enabled on your server. mod_rewrite is an Apache extension that is required for Omeka to work properly. Omeka is unable to check because your php.ini allow_url_fopen setting has been disabled. You can manually verify that Omeka mod_rewrite by checking to see that the following URL works in your browser: http://www.dcwarmemorialnetwork.org/install/check-mod-rewrite.html

What to do? TIA!!!

1&1 says to "contact the developer."

I should add that 1&1 verified that mod_rewrite is enabled. I get a 404 error if I try to go to admin.