Hi jack,
Welcome to the forums.
With the Windows version of the program, it's necessary to go into "Bar Settings" and then to set the number of beats for that bar to "2". After that, it's also necessary to use Bar Settings to set the bar immediately following the 2 beat bar back to 4 beats.
For example, say you want a 2 beat bar at bar 12. Click on Bar 12, enter Bar Settings and set the number of beats to 2. Choose OK. Now click on bar 13, enter Bar Settings and set the number of beats to 4.
As mentioned, this is using the Windows version. I suggest looking for "Bar Settings" in Help. I imagine that the process won't be too different.
Regards,
Noel