ThemeSumo on "about Post Loop"

Hi guys i really love this plugins. I have a question i know this question is already answers but its not working for me. anyway my question is about post loop i wonder how it work. on the bottom the. the additional input i put the category_name = my-category-slug but its not working im expecting to view specific category. example i have two category : 1.Counter 2.desk but i want to show in the post loop page is only desk is this how am i going to do that every time i do category_name = my-category-slug. but it's not working by the way im using twentey thirteen themes on wordpress thanks in advance guys.

Are you using a new WP_Query?
Does it work if you use the cat ID?

WP_Query: Category Parameters

Hope this helps.