short story: [showauto] won’t work as every other shortcode, so please don’t use it unless you have understood what’s different and where it works.
more details:
[showauto] is something different from the other shortcodes, it should be explained better in the documentation.
Apart from shortcodes, this plugin offers the ability to link a page to a category, in order to show the page content as header on your theme’s category template. This let you replace the usual ‘Archive for <category>’ with a more ample fixed header.
To make this work, you must define a page -> category relationship (hence the original plugin name: page2cat): you can do that on the “edit page” screen (look for a side box) or in the “edit category” screen.
Finally, when you have set up this exclusive relationship you need to edit the category.php template (or whatever it ships with your chosen theme) in order to display your page/header content: here’s where [showauto] is needed (and it’s the *only* place it should work). If you search in the forum you’ll find a lot of questions about this.
Here you should find some hints about the template modification.
Hope it helps!