Hi Rednil -
You can't link a category link to a custom page by default. You have two optoins for this.
The first option would be to put it in the left side link editor instead. There you can link to anywhere you wish.
The other option (more time consuming but possibly ultimately better) is to use HTML for your left hand navigation bar. You can view the source of the page, and copy out all the html which is created by everything you currently have that's using default MC tools. Then, you can very simply add another link under category which does not have the restriction of linking to only a category.
Hope this helps!