Just search on “extension manager will not open” in the Adobe Forums or Google and pages of hits come up going back to CS4. This is a serious problem that Adobe is simply ignoring.
I have Windows 7 Professional 64-bit with Design Premium CS5 installed. Nothing happens when I click on Manage Extensions from within any Adobe application, nothing happens when I click on Adobe Extension Manager CS5 on the start menu, and nothing happens when I double click on a .mxp file. No error messages – nothing.
So I had a free couple of hours and decided to call Adobe. Six hours later, after talking with Raj, Peenal, Tushar, Madhur, and Sandeep (get the picture?), after noting that none of them had ever even heard of the problem, after my call was dropped during one 30 minute wait, and after a complete uninstall and reinstall of the complete CS5, I have exactly the same problem I started with.
For my purposes, Design Premium CS5 is literally useless without extensions and Adobe has left me holding a $1500 boat anchor. Do you think anybody at Adobe cares? I don’t.
Bottom line - DO NOT BUY Design Premium CS5 if you need extensions. They very possibly will not install and Adobe won't/can't help you.