Originally we had implemented keyboard shortcuts as a user script. We have now finally made the change to have them integrated into the site.
When you go to the Edit Profile & Settings page on your profile, and then select the Preferences submenu, you’ll find a new checkbox labeled Enable keyboard shortcuts (which is located about half the way down on that page). Click it, and from there on, everything works as with the user script. Press ? to get started.
Once you enable it on one site, it's enabled cross-network.
If you already have the user script installed you don’t need to do anything. We detect the user script, disable its functionality, and run the built-in version instead. That means you don't have to manually enable anything, while still running the correct version of the keyboard shortcuts (currently the two are identical, but in the future, fixes and improvements will be made to the built-in version).