I love answering my own questions! For anyone else scratching their heads about this, here are the steps for adding additional compatible versions in the CKAN text-based UI:
Press 'F10'.
Select Game instance settings....
Press 'Tab' a few times to get to the Additional Compatible Versions list.
Press 'a' to add a new version.
Select version from list (1.10 for Spectra).
Press 'F2'.
And voila you should be able to find Spectra and install it. The original instructions for doing this are posted here: https://github.com/KSP-CKAN/CKAN/wiki/User-guide#Using-the-console-UI. However, it looks like they are for an older version of CKAN.
