The KarmaFX synth AU was not showing up in Reaper 64bit 4.402 (latest) on a Mac Book Pro, even though the installation went ok and running auval in terminal showed the AU was working fine.
Here is the solution:
Start finder (the smiley face icon on the left in your tray)
Then choose the "go" menu at the top of the screen and select "go to folder...".
In the pop up that appears type:
And press enter.
It should show a new window with the contents of that folder.
Locate the file:
And delete it!
Then start Reaper.
When you click insert instrument the file will be regenerated.
But more importantly, the synth will show up in your plugin list.
This seems to me like a bug in Reaper.
Nevertheless, until it is fixed perhaps this could help someone.
auval -v aumu KSYN KFX!