I just installed your plugin and it does not work at all. I am using the "U-Design WordPress Theme" theme from themeforrest. Your widget seems to work OK in admin, however whatever I chose I cannot get the widgets to appear on the website. I tried checking one of the categories for effect, or even the actual post in mind - no widget.
Not sure how I can point you to what is maby wrong here.
You should also think of adding a "close" button to your layer, and maby as posts and categories become many maby do not load the posts as the window will become HUGE. Atleast my needs are only category based.
To bad it didn't work :(