Been running an s3 with slush's pool for a month or so and accumulated maybe 20% of a Bitcoin. I started out using the gigantic Bitcoin Core Wallet because when I first learned about Bitcoin a couple months ago I had these big ideas about solo mining and a 25 Bitcoin chunk breaking off for me. Those were exciting days! Of course now I know I completely missed the boat on those days but I changed computers and do not want to download the core again. As soon as Slush's made a payout to me I backed it up to DropBox so I have that backup file but it isn't as easy as clicking a button in my new MultiBit wallet to recover my little piece of a bitcoin, Please Help! I'm going make a profile on here as soon as I post this question. I'll try to get username: Mickey-Mouse-Blacksmith
How to restore a backup made when using the big 20 Gigabyte Core Wallet to current MultiBit wallet?
First of all it is extremely dangerous to back-up your wallet to Dropbox or any other cloud based file service. You might find that if you do recover your wallet, it may have no more funds in it, but I'm hoping that you encrypted your wallet before backing it up to Dropbox and you can remember the passphrase.
Secondly, running vanilla bitcoind is not a mining version of bitcoin, by default, so I am assuming that you know what you are doing, although it might have cost you a considerable amount to run that on amazon.
Assuming that you haven't been unlucky or naive, you cannot directly recover your wallet to Multibit, the wallet file formats are not compatible but in any case this is not what you should be doing. The simplest way is to make a payment from your running bitcoind wallet to a new address in Multibit.
It actually surprises me just how many people don't understand this.