I have upgraded my machine form Windows XP to Windows 7 64bit and forgot to export my Actions. fortunately for me I did a complete image backup of the old Windows XP system before zapping it.
The theory is to find the actions in the Windows XP backup and copy them to the new machine.
If not "portable" installation - Mp3tag creates an automatic backup in archive:
%APPDATA%\Mp3tag\Mp3tagSettings.zip
If portable installation - find the archive in the Mp3tag $HOME directory
(e.g. C:\Program Files\Mp3tag).