As I am using Linux Mint 17.2. Following the instructions for downloading and signing the Tails OpenGPG key needs the user to install Nautilus-seahorse which is the extension for the Nautilus file manager to handle OpenGPG operations.
I decided to try and find out if there was a similar extension for the cinnamon nemo file manager.
According to this post on the mint forums. You need the nemo-seahorse package.
Firstly you need to search for the above package
$ apt-cache search nemo-seahorse nemo-seahorse - seahorse plugins and utilities for encryption
sudo apt-get install nemo-seahorse
Enter the appropriate password and it should then install.
At this point you will probably need to restart nemo. So I will be back later with part 2 of this post.