I tried to put some actions in the actions folder under the program's installation folder, thinking that would make all users to be able to use those actions, but it didn't work. What is that folder for if not that? Do someone have any tips to do what what I wanted in another way?
edit:sorry, should have been posted in the support forum... :\
Yep, I made this mistake before and took me a while to figure it out. You need to copy the action files you've created to a different location. On my machine it is:
documents and settings/your user name or all users/application data/mp3tag/data/actions
It is here you should copy the *mta files containing your actions. After this, reopen mp3tag and you'll see all the actions as available.
Sorry, but none of those posts answered my questions... 
Can I make actions accessible for all users without manually coping them to each and every single user's config directory?
(Not important, I'm only curious.) What is the actions folder under the installation directory used for?
It's only possible if you're sharing the same settings for all users. To do this, copy the mp3tag.cfg along with all other settings from %APPDATA%\Mp3tag to Mp3tag's program directory.
It's used as a template when a user runs Mp3tag for the first time. The files are then copied to the user's data directory.