It's not something you have to change. The IDLE command works on dreamhost imap servers, though they do not advertise or support it I don't think.
The way it works is your PDA's email client will send the IDLE command and keep its connection open to the mail server. If the mail server notices anything change in the mailbox (ie new mail delivered) while IDLEing it sends the details directly to the client. The client is also supposed to check in occasionally with the server to continue the IDLEing -- sort of "I'm still here, you're still there, ok let's keep idling..."
As you say it is not "true" push where the server initiates a connection from scratch, but it is very close and works great for me - I have my Treo set up with a mailbox at DH and it's pretty close to instantaneous between delivery to mailbox and delivery to phone.
Edit: sorry, I didn't totally answer your question re where to change the settings. I believe it should be totally transparent if the client and server both support IDLE. I haven't ever seen a preference or switch for it in an email client, and there definitely isn't one for the server side.