Hi dbdemigod,
welcome to the forums.
The recommended version at the moment is svn-1586 (this is the most stable "nightly"). Here's the download link for the Mac OS package:
http://nightlies.fireflymediaserver.org ... n-1586.dmg
You'll find the newer version svn-1696 here:
http://nightlies.fireflymediaserver.org ... n-1696.dmg
but there is no need to install that if svn-1586 works on your iMac.
Installation should be self explaining. If Firefly finds your iTunes library (and there's no reason it shouldn't), it should run "out of the box".
It is recommended to disable sharing in iTunes to avoid possible interference.
Greetings
dupondt