You first need to copy the encrypted file somewhere into your ubuntu home directory.
The first file after the
-in command option, named
'bitcoin-wallet-backup-testnet-2014-11-01' in your example, is the encrypted file you want to decrypt.
Navigate into the folder you have copied the file to and execute the command:
openssl enc -d -aes-256-cbc -md md5 -a -in YOUR_FILE_NAME > decrypted_file
Note that you might have to replace
aes-256-cbc if the forked client does use a different encryption algorithm.