It’s always best to use dreamhost’s name servers as the primary dns servers (see the link Ardco posted). It looks like your question is something like you have your dns hosted at SomeDNSHost.com or whatever, you want to keep it there and only setup the links on that dns host so that it will point to the proper location.
If the above is correct, then you have a couple different options. One is to manually set the correct ip address at SomeDNSHost.com. The other way, which is a whole lot easier if your dns provider supports it (and they should) is setup your dns service at SomeDNSHost.com to act as a secondary dns server. What this means is you setup the dns service at SomeDNSHost.com to secondary dns and the master dns server to 18.104.22.168 (the ip of ns1.dreamhost.com). You then have to get the ip address of your dns servers (just ping them if they don’t give them to you on the site somewhere) and put in a support request saying you want to setup a secondary dns for the dns servers (insert the dns server ip addresses). Support will then set it up so that when there are changes it will let those secondary dns servers know about it. You may want to send a support request first and wait till they say it’s setup before you setup the dns server at SomeDNSHost as a slave. Since as soon as you do that you’re dns is going to fail since it won’t be able to contact the master server until support sets it up. Also this means any custom dns settings you setup at SomeDNSHost are no longer going to work, you would have to send a support request if you needed a custom dns entry added. This is also all sorta irrelevant since you can just tell the registrar that you registered your domain name at to change the dns server to point to dreamhost’s.
The second method of just changing the ip address at SomeDNSHost to dreamhost’s ip is more simple, but if dreamhost ever changes the ip address, your site will be down until you change it. To get the ip address you just have to ping something.dreamhost.com that you got in your email. That is the ip address you’ll use for both web and ftp. Doing the mail service can be a little more complicated that I can get into if you really need to know. I’d just see about setting the dns servers to dreamhost’s servers though.