[Typo3] Why/where was Kasper wrong?

[Daniel Doesburg]

In the video of the CD collection, you learn how to make a selectbox to 
chose in the pagecontent one category. But when the page is displayed 
all categories are shown.

Did I miss something?

Or better, how can I make it that only the selected category is shown.

In the mailinglist is one thread about this subject. But I don't 
understand the explanation.

So, who can help me on the road again?

Regards,

Daniel