Depends on how you have the computer set up to recognize the second monitor
(a projector in this case). Here are the common scenarios:
1 - Computer set to clone output; what you see on the computer screen is
what is seen on the projector. This is probably the easiest and most common
method. (press F5 to start the presentation)
2 - Computer set for "extended desktop"; monitors act like on giant monitor,
what is on your computer screen is not necessarily going to be on the
projector. In this mode you can move you mouse off the right edge of your
computer screen to the second monitor. In this mode you need to tell
PowerPoint to play the presentation on the secondary monitor (SLIDE SHOW >>
SET UP SHOW >> MULTIPLE MONITORS >> make drop down menu SECONDARY MONITOR).
Press F5 to start the presentation. Slideshow will be on the projector,
while the computer monitor shows the presentation in the edit view.