I open the Administrator group, then press "Add..." The only thing in the "From this location" box is the local machine I'm working on, so I press the "Locations" button. In the resulting window, there is no other option to select from. The only icon in that window is the local machine.

Consequently, I cannot add a domain account to the local Administrators group.

So you want to add the local machine onto the Administrative group of a Domain?

Did you join the computer to the domain? If not, you need to do that before you can use domain accoutns for permissions.