you need to select those objects and use copy (Ctrl-C or copy in context menu on selected objects) then go to new diagram and use paste there. That will create copy of original objects.
If you simply want they to appear on another diagram then use drag&drop from browser
That was the first thing I tried.
It didn't work
We'll need more information to help you. What exactly are you trying to copy, and from different designs or just models in a single design? Logical or relational? Exactly what version of the Modeler are you using?
This tool is tthe sort of thing that emerges when "Kids in suits" don't do their job properly . Full of bugs , riddled with design faults and clearly not tested at all.
If you can't answer simple questions, we can't help you. Insulting us isn't going to get you very far either.