In this case, you need to set the encryption key on Express Scribe directly. To do that you may open Express Scribe, click on Options > Incoming > Set user's decryption key, then you need to set a user ID, and enter the Key they sent you to decrypt the file.
Once you have set the decryption key, you can try loading the encrypted key, and test it.
In case the issue remains, you may want to contact NCH Software support for further support.