Rated 5 out of 5 stars
I have used Thunderbird for many years and know how to install extensions. Have also modified the install.rdf file of other incompatible extensions and got them to install in Postbox 3.
Re- downloaded 1.0.9pre1 removed all extensions installed in Postbox and tried again - same problem. I'm using Win7 32bit.
I guess, there must something wrong on your side. Maybe there is an error in your profile. Try to setup a new one for testing.
I downloaded Postbox from here http://www.postbox-inc.com/download_success/win and installed it. Downloaded again the 1.0.9pre1 file from AMO (to test it), drag and dropped it into Add-ons window and it installed like on Firefox or Thunderbird. After Postbox restart the extension was fully usable, like my screenshot shows.