You don’t. PTR records (aka “reverse DNS”) don’t go in domain zone files. They have to be set up by whoever controls the DNS for the IP address range you want to modify.
If you have a dedicated IP address at Dreamhost, they might set this up for you upon request. They may not, but it can’t hurt to ask. If you’re talking about DNS for anything else – for example, if you want your static IP address at home to resolve to a hostname in your own domain – you’ll have to ask your ISP if they will set it up for you.
If you want useful replies, ask smart questions.