Under settings/wallet. Drop down list (if you have more than one wallet). It's good to make custom names for backups (wallets) - you will be lost very soon with "bts_default" naming scheme. If private key is from 2.0 it should work for you - I've tested some time ago. Can't help you with earlier version, because I had no idea about bitshares before, but if I had old key I would try to paste it somewhere under permision tab as owner key maybe. One I know older keys were not associated with accounts, so you need account first, then you can import the key,