This procedure is explained very well here, but it does miss out one very important point which can lead to a lot of balance missing after Multibit has finished synchronising (don-t worry though, it's not missing irreversibly!). The problem is that when you go through the process of getting the private key for each of your addresses and adding them to the file you're going to import into Multibit, you may be missing some hidden addresses that have been used for receiving change. These addresses are in your wallet, but are not listed in the interface. So my instructions following are based on the preceding link, but work based on your account labels instead of the addresses, you can then do an additional command to list all the addresses within each label which includes the hidden "change" addresses. The screenshot shows the process being performed on the "Bitstamp" receiving address, you can see that the interface shows only one address, but the console has listed two (you can click on it to see a full size version).
- Open Bitcoin-qt and open the receiving addresses tab
- Go to the Bitcoin-Qt console: click Help/Debug Window and then select the Console tab.
- Type the command walletpassphrase YOURPASSWORD 600 to unlock your wallet
- Type the command getaddressesbyaccount "LABEL NAME" for the first receiving address in the list
- Type the command dumpprivkey ADDRESSS for each of the addresses that show from the result of the last command (see screenshot to the right)
- Copy the results of dumprivkey and paste it into a text editor
- Do this procedure for all your receiving address labels adding the private keys to the list in your text editor
- Close Bitcoin-qt and start the Multibit client and create a new wallet if you have none.
- Click Tools/Export Private Key after selecting the Do not password protect export file option.
- Open the exported file in a text editor.
- Below the private key generated by MultiBit, paste the private keys you have copied above. Remember to paste them before the End of private keys line.
- After each private key, add a date that is prior to when you created the corresponding wallet. (You can check this date in the Bitcoin-Qt Transactions tab and scrolling to the very bottom.)
- Save the key file and import it to the Multibit client using Tools/Import Private Keys.
- In order to not leave private keys around, delete and trash the key files that you used.
- Et voila, you have just moved your wallet to the MultiBit client.