As I asked Joe, what type of watermark do you want? Do you know how to add a graphic to the gallery? ( Assuming your watermark is a graphic)
You begin with Format > Page. Click the Area tab . Select type of watermark from the Fill drop down list. Select the Transparency tab. Set the amount of transparency. Close the dialog.
The Fill drop down list is likely to be a problem because of the underlying choices that are possible. Custom colors can be created and used. Gradients can be created and used. Hash patterns likewise. Graphics can be added to the gallery and then used. ( This is done differently from how it use to be done. )
Dan