Hi Evalyn,
The right click , ‘cut opening’ option that Merv mentioned is NOT available in the default floor that is created when you enclose a space using the wall tool.
You must delete floors automatically created first. Then use the Floor icon from the Building toolbar selecting one of the options, most times I use ‘ by pick points’.
Once you have this ‘smart’ floor inserted, you can select it , right click your mouse and choose, ‘cut opening’.
Jack
Zimmer Design.com