Here is one way to accomplish what I needed (thanks to Robin Cruz for showing me):
1. Unzip the Rowgawski problems folder in /opt/webwork/Problems
note1: the file names were ridiculously long so I renamed the folder and zipped, then unzipped again.
note2: you might need to create the folder Problems
2. Create a symbolic link (pointer) within the Templates folder in the course you want to access these problems (or possibly modelCourse if you want access to the problems in any course created)
To create the link, go to the Templates folder and use:
ln -s /opt/webwork/Problems <pointer name>
for example, I renamed the Rogawski set as: Rogawski_Calculus so I used:
ln -s /opt/webwork/Problems Rogawski_Calculus
The problem sets are now accessible using the "local folder" button in the browser.