Actually, a better question is, do you have a 56k modem port on the iMac? That feature was phased out from the new iSight iMacs.
You have a built in 10/100 base ethernet and airport extreme if my memory serves correctly.
Apple computers are made under the "who still uses dialup and diskettes?" mentality. For a company that likes to stay in the latest trend, those things are obsolete and considered useless. Which is true to the demographic they target. I don't personally know anyone who uses an iMac and NOT have broadband.
If you MUST use dialup, you would have to get a USB 56k modem. One that has software that’s compatible with Macs. It will probably cost $50 or less. Depends on where you look and if you wait for a sale or not I guess.
But if the above is not the case. Here's an article from the Apple website about how to connect the internet via dialup.
http://docs.info.apple.com/article.html?artnum=106717