Dan,
I am hoping you have a working floppy drive on one of your 5300s. If this is the case, proceed as follows:
1. Download the two disk image files of the 3.0.6 Updater:
http://www.globalvillage.com/support/update-pp-56k-306.html
2. Download the GV PC Card Enablers Update Version 1.0.7 (a.k.a Updated Enablers for OS 9):
http://www.globalvillage.com/support/Updated-enable-os9.html
3. Not sure about this: "The latest update for your modem firmware is version 1.511 and is included in the 3.0.2 Updater." You can check your firmware version by clicking on the "How to check your current modem firmware version" in the link below. I am guessing that GV also included the firmware update in the 3.0.6 Updater but they do not say. The ZTerm utility should be in your GV folder.
http://www.globalvillage.com/support/powerport-platinum-pro.html
4. Once you have downloadedon your iMac, drag-copy each file to a 1.4MB floppy disk and transfer them to your 5300's HD. I am not sure, but you may have set your browser or Stuffit Expander to automatically open compressed files...you don't want this to happen on your iMac. If using Internet Explorer, go to IE's Preferences > Download Options > uncheck the auto decoding of BinHex and MacBinary files.
5. You said you don't have Stuffit Expander on your 5300. Perform a search with Sherlock under Apple menu for 'expander'. Do the same on your iMac.
6. You should have three files on your 5300's desktop...(2) .img files and (1) .sit file. Use Stuffit Expander to decode the files and just follow the instructions of the expanded files. According to GV, you want to run the 3.0.6 Update before running the Enablers Update Version 1.0.7.