It appears to be a bug affecting some users on El Capitan. https://discussions.apple.com/thread/7254177?start=0&tstart=0
There are two reported solutions:
- Reboot into safe mode to install extensions (not ideal)
- Use the keyboard to select 'Trust' rather than the mouse.
I think option 2 would be the way to go. Here's how to do that:
Go to System Preferences > Keyboard > Shortcuts. In the bottom section there is the text "Full Keyboard Access: In windows and dialogs, press Tab to move keyboard focus between:". Under that, select "All controls". Now the user can use Tab when the prompt to install the extension pops up. To submit the form use Spacebar (enter doesn't work).
Hopefully this gets fixed by Apple at some point.