While you can’t watermark your master photos in Lightroom, you can watermark copies as you export them, or as you create output in the Print, Slideshow and Web modules. Below is a video tutorial on using the watermark editor. It is one of 55 videos from my Lightroom 4: The Fundamentals & Beyond video series. (Update: You’ll also learn about this in my Lightroom 5: The Fundamentals & Beyond series – 12 1/2 hours of training for beginning and experienced Lightroom users, on organizing and editing your photos.)
As usual, for higher quality, once you hit Play, click on the sprocket wheel in the bottom right and choose 720/HD.