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The folding doesn't work at all

You may have a javascript conflict with jquery or mootools, which may be added to 
your theme header by your theme or another plugin. Try disabling mootools
in the file superslider.php

How do I change the style of the collapsing categories lists?

I recommend that you move the folder plugin-data to your wp-content folder if you already have a plugin-data folder there, just move the superslider folder. remember to change the css location option in the settings page for this plugin. Or edit directly: wp-content/plugins/superslider-menu/plugin-data/superslider/ssmenu/custom.css. Alternatively, you can copy those rules into your WordPress themes, style file. Then remember to change the css location option in the settings page for this plugin.

The stylesheet doesn't seem to be having any effect?

Check this url in your browser: http://yourblogaddress/wp-content/plugins/superslider-menu/plugin-data/superslider/ssmenu/custom.css If you don't see a plaintext file with css style rules, there may be something wrong with your .htaccess file (mod_rewrite). If you don't know how to fix this, you can copy the style rules there into your themes style file.

How do I use different graphics and symbols for collapsing and expanding?

You can upload your own images to http://yourblogaddress/wp-content/plugins/superslider-menu/plugin-data/superslider/ssmenu/img_custom

I can't get including or excluding to work

Make sure you specify category names, not ids.