So if your domain is fizbin.com, for example, it would be mail.fizbin.com for incoming and outgoing in Apple Mail. I assume that you set up your site in DreamHost’s Web panel to point to their nameservers (ns1.dreamhost.com, etc., etc.) and to allow DreamHost to host your mail, right?
If the DreamHost end of it is set up properly, your settings in Apple Mail for Lion should fall right into place. Let me know if you need any guidance on that end. And, if you upgrade to Lion’s successor this summer, Mountain Lion, what you set now carry through without alteration. Trust me.