Got it . In case someone else need it :

    Run Core Wallet
    Select Help (to the right of Settings)
    Select Debug Window
    Select Console
    If you encrypted your wallet, unlock it by entering walletpassphrase "YourLongPassphrase" 600 next to the > input box
    Next to the > input box type importprivkey (Note that the private key must not have spaces, remove them if they are included in your backup)

Example: importprivkey 5KJvsngHeMpm884wtkJNzQGaCErckhHJBGFsvd3VyK5qMZXj3hS