And burners who wanted a quick profit have already sold for weeks...
Not me

Where can I download a Mac client to import my private key?
Same question (it is a bitcoin private key, on which I also have some BTCs, hope that's not a problem)
A standalone Mac wallet is not available. One will be available in the future, but in the meantime, go to Counterwallet.co, generate a new wallet using a mnemonic passcode, and "sweep" your XCP (and a bit of BTC for fees) into your new Counterwallet address. This will require your BTC private key(s) for a one-time use. After that, your XCP will be in separate addresses from your Bitcoin wallet addresses, and you will use the Counterwallet to manage all Counterparty transactions.
Eventually, full-node clients will be introduced, but at this time it's rather complicated because of the moving parts between bitcoind and counterpartyd.