I've also got Macromedia Extension Manager version 1.7.240 installed, which works with my installation of Dreamweaver 8. It's what I'm using while my CS4 is messed up. Might there be a conflict? I'm afraid to uninstall it because I'm not sure who in my company has the original disks, in case I need to install it again.
I had the exact situation with Vista and this is what I did after seeing something on another thread that gave me a crazy idea:
The thread mentioned installing manually into a user directory. (Users/[name]/Library/Preferences/Adobe/Extension Manager2/Configuration/Extensions/
The problem was that I did't have the "../Configuration/Extensions/" folders, so I created them and threw an .mxp file in (while every CS4 program was closed).
I then started up the CS4 Extension Manager and instead of freezing and being blank, my extensions showed up and I was able to install again. Maybe just a fluke on my end? I would be interested to know if it works for someone else.
I'm having this problem but on Windows 7. I've tried adding the folders as suggested but alas this has not solved the problem for me. From within Dreamweaver, Extension Manager opens but freezes and does not populate with any entries. When I double click the .exe directly where it resides, it does not respond at all (both as a user and administrator).
I also had an additonal problem similar to the OPs where right clicking fails in Windows Explorer accompanied by the following message:
The specified module could not be found. C:\Program Files\Common Files\Adobe\Adobe Version Cue CS4\Client\4.0.1\VersionCue.DLL
I solved this by moving the file mentioned in the error message from the 4.0.0 folder to the 4.0.1 folder (also mentioned in the error message) - not my bright idea, but it worked, although perhaps it lead to my new problems.