Edith

The default shortcut to confirm a segment is CTRL-Enter. If Studio suggests ALT-Add, it means that you chose the wrong user profile when you launched Studio the first time. You chose "SDL Trados" instead of "Default".

You should change your user profile because this not only affects the shortcut for confirming segments, but also others.
To do this, go to "File - Setup - Manage User Profiles". It will show "SDL Trados" as your current profile. Click on "Change user profile", in the next screen on "Do not backup ..." and select the profile "Default" in the next screen.
Click on "Finish" and you will now be able to use all the default shortcuts.

BTW, the "Add" key is the + key on the numeric pad. This setting "SDL Trados" is meant for users who want to stick to the shortcuts that were used in old Trados Workbench (many years ago).

Walter

This was useful. Just wondering why the starter interface suggests you SDLTrados as a profile for known users; after so many upgrades, I don't consider myself as a new user. Might be a good idea to find another name and explication for those options