If you and your wife are both running the same OS X version, and same version Safari, you could manually drop the extension in the proper location on your Mac, then launch Safari. The extension should then show up in your Safari Preferences/Extensions. (Correct location will be ~/Library/Safari/Extensions folder) That folder, again, is in your own home folder. The Library folder for your user is normally hidden, but easy to get to from the finder, by clicking the Go menu. You will see the link to your Library appear when you press and hold the Option key.
Easiest way to get the extension is to copy to a flash drive from that location while logged in to your wife's account, then copy off the flash drive when you are logged in. Make sure that Safari is NOT running when you copy the extension.
If you STILL have problems getting that to work - are both you and your wife's accounts listed as admin accounts?
You can double-check that in the Users & Groups pref pane.