a little bit more detailed explained.

1. right-click to the desktop then -> More --> Enable Folder Actions

2. in Terminal App open the window /Volumes
sudo open /Volumes

you will get a finder window opening showing the mount point /Volumes

3. right-click on /Volumes -> More --> Attach a Folder Actions and select the script
/Library -> Scripts -> Folder Action Scripts -> new item alert.scpt

4. plug in a device (memory stick for example) and see what happens.

hope this helps
blackwire