Help:How to import private keys in Bitcoin Core 0.7+
Open Debug Window
Then go to menu: /Help/Debug Window
and click on the tab - Console.
Unlock your wallet
If your wallet is encrypted (I hope it is!) you must unlock it. If not just skip this step.
To do this just type into the box at the bottom:
walletpassphrase YourLongPassphrase 600
The 600 means your wallet is unlocked for 10 mins (600seconds
Run Import Command in Debug Window
In the console at the very bottom is a text entry box in here enter:
importprivkey yourPrivateKeyInWalletImportFormat TheLabelThatIWant
You now have to be patient. On a fast PC it takes 2 minutes to import, and during this time it looks like you application has hung. After waiting a few minutes you will see:
You are now done. But always best to check it worked.
Check Key Imported OK
Once Imported you can check that you have the address by closing the Debug window and going back to your address book.
You should see the address here:
And you can even send a transaction to check e.g.
Your backup of you wallet will not have this key in obviously. So before you do anything else close the client (to ensure that it is locked again) and backup the wallet.dat file as normal.