If you like to see only those items that are placed under specific folder in the folder hierarchy, you can do it as described below.
under:[name of the folder]
After typing 'under:', you should enter the name of that folder. For example, if you like to see only those subscriptions which are located under the foler named 'science', you can enter 'under:science'.
Keep in mind that while checking given folder name against existing ones, Newzie ignores case sensitivity. In other words, filtering command 'under:Science' is no different than 'under:science'.