I had bitcoins on MultiBit 0.12 on my old computer. I have moved the old computer's hard drive to an external hard drive. How can I transfer my bitcoins from the old hard drive to my new computer's MultiBit 0.15?
MultiBit developer here. The version numbers you're supplying are not related to MultiBit. We currently have version 0.2.0 for MultiBit HD and 0.5.19 for MultiBit Classic. There has never been a 0.12 or 0.15 version. Are you sure that you have the correct wallet application?
That aside, if you need to empty your wallet in MultiBit Classic, you simply need to use File | Open to locate your wallet file, and then use the Send tab to transfer those bitcoins to another address. That address is provided by whichever wallet you'd like to use instead.
Do not delete your old private keys in case someone sends you bitcoin to an old address
Here's a help article that gives more details: https://multibit.org/en/help/v0.5/help_sendingBitcoin.html