You need to implement two mockups for Collapsed and Expanded states of the menu, see the image:
To provide expanding behavior on click in Presentation mode, set the link to Expanded view in the Collapsed view, see the red arrow in the File item.
Balsamiq implements “between page” prototyping model, i.e. each state is designed on separate page. Other tools, like Axure RP, provide “within page”, or “layer prototyping” model, where the states can be implemented within the same page.
Still, you can use some ninja techniques in Balsamiq to convey the interaction on the same page, but the separate pages for each state is the easiest and straightforward way.