Open the Project so that you can create a WBS for that particular project. Right click the name of the project and click open. Then once the project file is open, from the top Menu click Project and select WBS then the WBS Tab Window will open.
To add WBS click the + icon at the upper-right screen so that you can add WBS items. You can also click the right, left, up, and down icon to indent and unindent a WBS item.
After adding all you WBS items, you can go back again to Activities Tab window and add new activities under each WBS. You can also cut and paste activities to transfer them to their proper WBS categories.
Here is a video for more explanation about WBS.