Hi Nickals,
As Darshan said, in Trimble Connect Desktop drag and drop is possible on the project data page, but not in 3D view. (I.e. not in Models tab.)
If you drop model to data page, in explorer, then switch back to 3D using the 3D view image under Views. In 3D use the "Model selection" in Models tab to bring the newly added (or any other) model to the models tab.
Hopefully this helps.
Best regards,
Marko Myllymaa