How can I copy and paste elements?

Mockingbird has full keyboard support for cut, copy, and paste, so you should be able to select an element (or multiple elements) do Cmd/Ctrl-V for cut, Cmd/Ctrl-C for copy, and Cmd/Ctrl-V for paste. You can even paste on a different page and the elements will be pasted in the same place as on the original page.

If you prefer to use the toolbar instead of the keyboard, you can click the gear button in the toolbar and use the cut/copy/paste options there:

Cut, copy, and paste from the toolbar using your mouse