On the IPad go to Settings -> Mail, Contact, and calenders -> add account -> Other
Pick IMAP if you get a choice for POP3/IMAP...
For the hostname use mail.YOURDOMAINNAME.com
for the username use the FULL email address.
Even tho it may claim on the IPAD that feilds outgoing smtp server settinsg are optional, they are not, you have to set it up. use the same hostname, full email for user name and password.
If you leave SSL off you will get it working faster, but if you want a secure connection to the server the incoming port you need is 993 and the outgoing port is 587
Also once you can send and receive mail, it's helpful to go back into "advanced" in the settings and set the Drafts Mailbox, Sent Mailbox, an Deleted Mailbox to the appropriate server folders.
hmm someone has made a nice wiki page for this too. It's shown as iPhone, but iPad is the same: http://wiki.dreamhost.com/Iphone