VM:Getting Specific Version
You can always upgrade your VM to the latest current stable release using the script provided (see here).
If you want to run a different version because you're madly tracking development releases or want to go back to one that worked then you can download and install it manually.
Login to the console as root
First go to to the /tmp directory
cd /tmp
Now get the version you need. You can find the download link by going to the release page from the Show Previous Versions section of the website (note this is usually http://www.purplepixie.org/freenats/downloads/freenats-X.XX.XR.tar.gz i.e. http://www.purplepixie.org/freenats/downloads/freenats-0.02.25b.tar.gz for 0.02.25b).
Use wget to download the tarball file
wget http://DOWNLOAD_URL/DOWNLOAD_FILE
for example
wget http://www.purplepixie.org/freenats/downloads/freenats-0.02.25b.tar.gz
You should see some output indicating the download was a success.
For the rest of this document we'll assume the version you downloaded was 0.02.25b
Extract the tarball
tar -xzvf freenats-0.02.25b.tar.gz
You could see lots of output showing the directories being inflated.
Change into the directory and run the vm-install script
cd freenats-0.02.25b ./vm-install.sh
This will take you through the install of the version you have downloaded.
To clean up afterward
cd .. rm -Rf freenats-0.02.25*