Please find attachments icon in the message header and click it. Select the registration key file (for example, AccelClip #2-01-232123-234.fxregkey) from dropped down menu.

You can see the confirmation message, that inform you about potential danger of opening files from unknown senders. Please answer 'Yes' to open the registration key.

Now the registration key is installed and you can see corresponding notification on the screen. Please restart application to complete the registration.