I'm having trouble with the T.O.O.L. mod

So I have been trying to use the T.O.O.L. mod to duplicate a group of objects and then move them as one but I can't seem to select multiple objects, I know that you can but it doesn't seem to work. When ever I select another object to highlight it as well, it highlights the new object but the highlight on the original goes away. I'm not sure what I'm doing wrong.