I did find this link, but it is for CS5. I tried it replacing the CS5 with CS4 and it did not seem to work.
If your Watermark add-on has an .mxp extension, you can try right or Ctrl-click on that package and direct it to "open with" the CS4 extension manager.
I'm running Mavericks and that's how I installed an extension file just recently. There were no admin errors.
Mid 2009 Macbook
My watermark is an .mxp. The problem I am having is after I open try to open the watermark in extension manager by ctrl-click on the package. It is at that point i get the "I dont have permissions" window.
Can you tell me what watermark you are using? Maybe this is a watermark problem and not an extension manager problem.
I'm not using one at the moment. My extension is a Channels and Masks panel. You do bring up an idea. You could go to the Watermark vendor and see if there is an update or if they have an idea what is causing this,and how to fix it.