As noted here, PUMA is a state-dependent variable, therefore in order to obtain information about a particular PUMA, you will need to specific the PUMA code as well as the state code.
Using the map on this page, you can identify the 2010 PUMA for the area in which Harrisonburg, VA falls. Type Harrisonburg, VA into the search and you will find that the 2010 PUMA for this area is 51110.
The STATEFIP code for Virgina is 51.
Using puma(51110) and statefip(51) will allow you to conduct your analysis using the online analysis tool. For example, if you are interested in the gender breakdown of this PUMA, you can specify the following:
filter: puma(51110), statefip(51)