"Show in Finder" doesn't work

It's been working for me. Something might be wrong with your system.
Have you tried logging in as a different user and see if it works there. That will help determine if it is system-wide problem or just your user account.

Yeah, that's the difficult part. I don't know where that would get corrupted, so it's difficult to guess what to do.

Does it try to do anything when you select it? Like switch to Finder at all?

I'm still at a loss as to where it could be, but perhaps the Dock.

First, have you restarted since this has been happening? If not, try that.

Then, try resetting the Dock by deleting the com.apple.dock.plist and com.apple.dock.db files from your user/Library/Preferences folder, then immediately log out and back in.
You'll have to rebuild your Dock to your liking.

If that doesn't help, open up Console (in Utilities folder), select the All Messages or Console Messages and then try the Show in Finder command. See if any related messages pop up in the Console.

sudo killall -KILL appleeventsd

fixed mine, thanks Baze.