Copy your private key into a file, eg privatekey.txt
In Linux,from your Ethereum-Wallet subdirectory, type
./resources/node/geth/geth account import privatekey.txt
In Windows,from your Ethereum-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 and the new account will be loaded into the wallet.