Copy your private key into a file, eg privatekey.txt
In Linux or Mac, from your Ethereum-Wallet Wallet (Mist) subdirectory, type
./resources/node/geth/geth account import privatekey.txt
In Windows, from your Ethereum-Wallet Wallet subdirectory, type
resources\node\geth\geth.exe account import privatekey.txt
In both the above options, you will have to enter a password twice to encrypt your new account information.
You can now start Ethereum-Wallet Wallet and the new account will be loaded into the wallet.
Remember to delete privatekey.txt after you have successfully imported your private key