Sounds like you just bought IndexU 5. I can't believe they are selling a product that can ONLY run on machines that have mod_rewrite enabled.
Did they even notify you that it was required before you bought the script? If not then do a chargeback and purchase something else. The lack of official support is frightening, most support on the forums is done by members.
Basically what this user is asking for is to have mod_rewrite enabled on his server.
Do you run your own server? If so you would need to ssh/telnet in and add the required lines to your conf files.
Do you have a shared server? Ask the admin if they will enable it for you. If not, find a new host.