Example: importprivkey 5KJvsngHeMpm884wtkJNzQGaCErckhHJBGFsvd3VyK5qMZXj3hS
The label may have spaces, the quotes are only needed if you want a space in the label.
On a fast PC it takes 2 minutes to import, and during this time it looks like you application has hung. You are now done.
While adding multiple keys you may want to avoid a time consuming rescan after each key. To do so use the command importprivkey bitcoinprivkey '' false for each key and importprivkey bitcoinprivkey '' true for the last one. The blank label is needed to pass the true/false flag.