Here is a summary of how to do this. Special thanks to Nick and Jonathan for helping out, and Jeff for the button pic.
I should note I am using Wildfire 3.0.
I also included the directions on how to 'undo a break', along with a picture of the button you can use from the toolbar.
TO CREATE A BREAK:
Insert -> Break... (or use the button) and then click on the witness line where you want the break - click the start and end points. There are also the options 'simplified' or 'parametric'.
TO UNDO A BREAK:
Select the dimension, and then point at one end of the break and right-click. 'Delete' in this context-sensitive menu means 'delete break' - there's also a 'delete all breaks' command depending on the number of breaks a witness line may have.
Good luck‼
~Brian