First you need to send the Sender your Public Key.   You are the Recipient.
Here's how:
Open Away RJN Cryptography and click on 1 - Send Public Key.
This form will open:

When you click OK, your Public Key will be generated and Mozilla Thunderbird will open like this:

Click "Send"

When you receive the email with your Public Key Encrypted file attached, just Save it to one of your Special Folders and Decrypt it using the Session Key that the Sender has sent you.

Notice how the red/blue icon shows the file as Publically Encrypted.

When you click the DECRYPT button, this form shows up.

Away RJN Cryptography will then Decrypt the file

 

Your Special Folder will change to this:

Your file is Decrypted and ready for viewing. You can now encrypt it with your Private Password at your convenience.