The only person who can set a reverse DNS is the owner of the IP addresses, such as the service provider who purchased them in large block quantities. I don't think it's something most internet or server customers have access to. You have to open a ticket with your provider and ask them to do it for you on their systems.
A description of the problem: http://support.simpledns.com/kb/a100/how-do-i-get-reverse-dns-for-my-ip-addresses-delegated-to-my-own-dns-server.aspx
A technical explanation of how your provider would handle this on their end, if they wanted to give you full control: http://support.simpledns.com/kb/a146/how-to-sub-delegate-a-reverse-zone.aspx