TL;DR: Drag and Drop in Mockups 3.4.5 (and below) doesn’t work in macOS Sierra. Download Mockups 3.5.1 here.
As you may or may not have noticed, Mockups has some trouble with Drag and Drop when running on macOS Sierra. This is due to a bug with the version of Adobe Air that is bundled with Mockups. This bug affects all versions of Mockups 3.4.5 and below.
The good news is that the issue is fixed in Mockups 3.5.1, which you can download here.