PokerTracker

When I observe a table HUD does not show on the table nor does it "read" played hands? Is it a bug or does it only work when I sit? I did not try to sit because I currently don't have money in my account.

There are no played hands while observing. Most sites do not allow for the import of any observed hands, though you will be able to get 30 observed hands per session at PokerStars if you enable the Memory Grabber option on our PokerStars Configuration tab. Once you have any hands in your database, you can also see the HUD on the replayer while the auto import function is not running.

Can you explain me what memory grabber does?

It checks the memory of the PokerStars process in order to read the recent hands in accordance with the PokerStars terms of service.