Can't add portfolio categories to menu?

Hi forum people,

Struggling with the ‘new’ Skylab theme. Bought the original version some years ago. Rebuilding the site now on a new platform… Back to the question: I’m probably overlooking something here, but I can’t seem to add portfolio categories to a menu. Blog categories seem to appear automatically in the menu control page. But can’t find the portfolio categories there. Must be possible though… What am I overlooking? Please advise…

ps. I tried to communicate about this with the good people from ThemeForrest and Skylab but they are practically forcing me to buy the theme again or buy into their support plan because support on the theme expired. For the seemingly simple question that I have, I thought I’d give this forum a chance first.