[Typo3] How to show only one category in CD collection tutorial?

[Fabian Raschke]

Hello,

I watched Kasper´s CD collection tutorial. He´s extending tt_content to 
make it possible to select cds from a specific catagory as a content 
element.

Unfortunately he´s not telling us how to code it at the end. If you 
don´t do anything you can select the category but always ALL categories 
show up. So some code must be missing.

I searched the archive and found paople asking the same thing but all 
have been unanswered. Can anybody please help with this?

Thanx,

Fabian