ELI5: How do I import keys from blockchain.info backup into MultiBit 0.5.18
Support for importing blockchain.info wallets was removed in MultiBit 0.5.18. MultiBit still hosts their old releases, however, so you could simply download 0.5.17, import the blockchain.info wallet, and then upgrade to 0.5.18.

You could also import the backup into a new blockchain.info account. Once it's there, you could send it to a MultiBit address or make another backup in an easier-to-read format (e.g. unencrypted export). You could then follow the instructions here to import those private keys into MultiBit 0.5.18.