To display only one folder in a Document and Media Library portlet, select the desired folder as the root folder by clicking the spanner icon, choosing "Select" under Root Folder, and then choosing the folder. After selecting the folder, scroll down and click "Save" to display only that folder in the portlet.

1. Displaying a Specified Folder ONLY
In a Document and Media Library
Instructions for if you want just one folder to be showing in a Document and
Media Display Portlet are as follows:
Step 1: Click on the spanner on your document and media display.
Step 2: Under Root Folder, click “Select”.
Step 3: Find the folder you want to display and click “Choose”.
Step 4: It will then take you automatically back to the previous page, and you just
need to scroll down and click “Save”.