Comment 3 for bug 1014277

I'm fairly sure this isn't a duplicate of bug 911706. Bug 911706 is not about computers that aren't connected to the Internet; it is about computers that are connected to the Internet without using Network Manager.

Judging from the linked tutorial, the fix for this bug is to enable the "Install" command not just if there's an Internet connection, but also if there isn't an Internet connection but the selected package is from a file:/// repository.