In my case I put them manually, Just like this:
<albums>
<album title="All Photos" description="Pixelpost Album" type="mediarss" url="http://www.i-marco.nl/photoblog/addons/mediarss.php"/>
<album title="Abstract" description="Pixelpost Album" type="mediarss" url="http://www.i-marco.nl/photoblog/addons/mediarss.php?category=9"/>
.
repeat the upper line for each category
.
</albums>
You can find out the number for each category in your browse page.
Jut put the mouse over the category and see the number on the foot of your browser.
lfp