Found my multibit.key file, what now?

I used BitCoin for the 1st and final time so far about 2-3 years ago. I thought I had lost my wallet but I found an old backup of my hard drive and it contains a file “multibit.key”. I installed a program called MultiBit HD but all the instructions for importing an old forgotten wallet are for the old version of MultiBit.

So in short: what now? I would like to open my “multibit.key” wallet, I think I remember the password but I have a hard time opening/importing it to MultiBit HD. If that is even the right program to use… 🙂