"DVD or CD Sharing" blocked by Firewall. How to unblock??

Hola there! from a MacBook Pro on OS X 10.6.4

Straight to the point:

On System Preferences > Sharing, I have the "DVD or CD Sharing" checkbox ticked, i.e. Service = On, however on the right hand side of the pane the on/off dot is red and it reads "DVD or CD Sharing: Blocked by Firewall".

Problem is that on System Preferences > Security > Firewall > Advanced…, "DVD or CD Sharing" appears set to "Allow incoming connections".

So what am I missing here??

In case it helps, the options on Firewall's Advanced pane I've tried are:
- "Block all incoming connections" - UNchecked
- "Automatically allow signed software to receive incoming connections" - checked
- "Enable stealth mode" - both checked and unchecked.


Muchas gracias in advance for your much needed help!

Alberto