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... :)
MultiBit developer here.

You'll need to use MultiBit Classic version 0.5.19 available as a download from the site https://multibit.org. This will allow you to open the .key file and synchronise with the block chain to recover all the funds associated with the private keys held in the file.

Once you have imported the keys, we strongly recommend that you upgrade to a HD wallet. The instructions to do so for MultiBit HD are here: https://multibit.org/en/help/hd0.2/how-to-upgrade-from-classic.html.

Essentially, you download MultiBit HD (the two can sit side by side without interfering with each other) and then spend from Classic into HD. 

Always keep hold of your private key files in case someone spends to an old address.