Help > Debug Window > Console
Next to the > input box type:
importprivkey

You'll need to unlock your wallet before you can run this command if you've used a passphrase:
walletpassphrase 60

The 60 at the end unlocks it for 60 seconds.

Source: https://www.reddit.com/r/BitcoinABC/comments/6q747s/in_bitcoin_abc_client_where_can_i_import_my/