Allain,
Just figured out the fix. You have to go into each of the office programs and look at the "Add-ins" under the "Options" menu. There select Nitro, which is "inactive", make sure "Com Add-ins" is selected next to "Manage" and click on "Go". In that menu, check Nitro and click "OK". Do the same for the other Office programs and Nitro shows up in the ribbon!