Enabling Folder Actions Within OS X?
Firstly, bear in mind that Folder Actions can be configured through AppleScript to perform a huge variety of actions, and that there are some default scripts among which you'll find the adding / removing action you mention. 

Since 10.6 the Folder Actions Setup panel is accessible through any folder's contextual menu:


Do Control+click (or right click) on any folder for the contextual menu to show. 
Choose Folder Actions Setup. ( If you've got any eligible services already, it would be slightly different as you'll have to go to choose Services and then Folder Actions Setup.)
Choose the add - new item alert script from the prompt window.
Select the Enable Folder Actions option in the upper-left side of
the setup's window.


You'll have, though, to edit the script for it to do something useful as, right not, it only displays an alert saying that an new item has been added to the folder.