Airport extreme connect to external hard drive.

Please check out the following Apple Support article for details on how to access an AirPort Disk from either a Mac or PC.

Read the article before I asked the question.

Sorry, I didn't assume anything here. You did not provide much detail in your post on what you have already tried to do to resolve this.

I did not get a CD with the Airport Extreme. No Airport Disk Utility on the iMac. Ordered the Airport new, not a referb.

I'm surprised that a CD was not included with your new AirPort Extreme. The AirPort Utility, that comes on the CD and is included in a number of OS X builds, replaced the AirPort Admin Utilty and the AirPort Disk Utility as they are now both integrated in one application. You can also download a number of versions of the utility directly from Apple here.

Finder sees the Airport Extreme as a shared drive but I cannot get the new drive icon to appear. This is a Segate GoFlex also new.

Typically if you can see the drive in the Finder, then the AirPort both recognizes it and is sharing it out. Does the drive show up properly in the AirPort Utility? The AirPort cannot share out drives that are formatted in either NTFS or ExFAT. Is it possible that your GoFlex is formatted in either of these?

Ok, two things to take into consideration:

1. The format Time Machine uses when the HDD is directly attached to the Mac vs. when it is an AirPort Disk is different. Time Machine would need to start a whole new backup. Please check out this Pondini article
2. Unfortunately, Apple does NOT support Time Machine backups to AirPort Disks attached to the AirPort Extreme. If I would have known that this was your original goal, I would have warned you not to. Pondini also discusses this as well in this article.