Please move this thread to someplace more technical, like the
"Alternative clients" board, more people could help there.
There were Multibit and Multibit HD. Please make sure about what did you use and what you installed now.
Iirc, Multibit (Classic) used to store its wallets on user profile folders. C:\Users\<your_user>\AppData\Local\multibit or C:\Users\<your_user>\AppData\Roaming\multibit or something like that. I don't know the equivalent on Mac, but you have to find the wallet file, I think it had the extension .wallet; more files, like .info and such *may* help, but the wallet file or a backed up .key file is want you need. Without that you may have just started Multibit which created a brand new wallet (with brand new address).
Btw, don't you have written down the old address with your money on it? One thing would be to see if this wallet you have now has the same address...
So.. first of all, you should start by searching on the old hdd for files/folders called multibit or *.wallet...