To add a stopwatch in a PowerPoint presentation, go to the Slide Show tab and select Ser Up Slide Show option from the Set Up category. In the option window that just opened make sure that you have selected the Using Timings, if present option. It can be found under Advances slides. Click OK to close the window and click on Rehears Timings. Your presentation will now go to full screen and you can manually set up the time for each slide.
If you want to add a virtual clock in your presentation with a ticking sound, you can view the following forum where this problem has been discussed.