I'm an occasional GIMP user and very new to the plugins. I'd love to try the Wavelet Decompose Plugin but don't know how to get it installed and running on my Mac OS X 10.6.8. I'm currently using GIMP 2.8.3. Any help is greatly appreciated.
There's a couple of ways, but honestly it might be easier for you to pickup a pre-built GIMP that already has it included. I can attest that Partha's GIMP builds are great for this, and include many extra plugins that you may find useful (http://www.partha.com).
I personally just use his builds these days, and I am also on OSX.