Ok, am I understanding correctly that Mobipocket is not compatible with a Mac? What do we Mac people download these books with? {I do have Stanza on my Mac}
You do not need anything special on your Mac to download these books. Just go here
http://www.starrigger.net/Downloads.htm and download the Mobi/Kindle version of each book. The downloaded files will appear in your Downloads folder on the Mac.
Hook up your Kindle to the Mac with the USB cable that came with your Kindle. The Kindle will appear on your Desktop as if it were an external drive (it is).
Double click on the Kindle icon to open it and you will see a folder in it named Documents. Drag each downloaded book file from your Downloads into the Documents folder in you kindle and drop it there.
"Eject" the Kindle (unmount it). To do this, single click on its icon, then look under File in the Mac's menu bar for "Eject Kindle" and click on that action. As soon as its icon disappears from the Desktop it's OK to disconnect it.
The books should now appear on your Kindle's Home page.
Hope this wasn't too detailed an explanation but I wasn't sure how comfortable you are with your Mac or which version of the Mac operating system you are using. PM me if you need more help.