There are a couple of reasons why that option to import from Papers 3 may not show up. 

Please start by checking that Papers 3 is located in Applications on your Mac. It must be located in Applications and it must be able to launch and display your library.

Next, head into the Privacy & Security Settings on your Mac and check that both the new Papers and Papers 3 have automation access

If you do not see both apps in this area, please reboot your Mac and try again.