Step 1: Select "Tools -> Import Private Keys"
Step 2: Click the "Choose import file" button
Step 3: It should prompt you for the password, enter the password and click "unlock":
NOTE: if the password is incorrect, you'll get the "The private keys unlock failed. The error was "Could not decrypt input string."
Step 4. Import the keys:
If the password was correct, it should show a count of how many keys are in the .key file and the 'replay date' (this is when they were created)
Then simply click the "Import private Keys" button. You should see the "success" message:
You will likely then need to wait for Multibit to finish syncing the wallet... it should still work, it will just be very slow as there aren't many peers any more. Once it has finished syncing, you should be able to export using the "Tools -> Export Private Keys" option:
NOTE: DO NOT password protect the export file... otherwise you'll be back where you started