Not sure if anyone noticed this before, but if you have used all of the categories for a project (15 plus an "unfiled" category), you run into a problem with the flat filter.
The flat filter "Categories" button brings up a screen that lets you select which cats to include. There is supposed to be a scroll bar on the right, but it does not work and is not displayed. This is a big problem with my 15 or so Getting Things Done (GTD) @context categories.
Without changing the source code, however, I was able to make a simple change on-the-Palm using RsrcEdit. I reworked the Select Categories form so that you can get the full list of up to 16 categories displayed.
The attached zip file has screen shots of the old form, my new workaround form, and the my new PRC file (if anyone's interested).