I would also like to know this. The basic instructions assume everyone is using Microsoft. There is no "right click" on an Apple product, and nothing I have done thus far will change the default 2 duration.
The Multimeasure rest dialog is used to set the duration of a rest in measures.
To Access this dialog. After setting a measure to be a multimeasure rest right click on the number above the measure and select "Set Rest Duration" from the context menu.
To remove a multimeasure rest either: set the duration to 0, or right click on the duration and select "Remove Rest".
Number of measures:
The Length in which to rest for in measures.