I believe it is in the actual settings for the instance of the module that you place on your page.
Place the 'Alicommerce Product List' module on your page. Then edit the settings of this instance (most likely with the drop down menu or other link).
At the bottom of the module settings page you will see an option for 'Display Settings'. Expand this and you will see the select box for default category.
If you want to create a specials, I believe you may have to set up a new category to accommodate that. Then make that the default category for your Product List.
I have not tried this, but you may be able to hide the category so that it does not show up in the category menu on the left but you could still display all of the products in that category as your default.
I have just started working with Alicommerce a week ago, so there may be a better way. But this should work for you.