If you are not able to project your desktop through the projector follow these steps.  

Under the Apple menu in the upper left corner drop down to "System Preferences".  

Once system preferences opens select the "displays" option. 

Make sure the computer sees that a projector is connected.  
You can tell because in the "Displays dialog box there is a tab called "Arrangement"

If the Arrangement tab is not there. 
First make sure the projector is turned on, 
then make sure the connections from the projector to your desktop machine are tight.

Click on the Arrangement Tab.
Check to make sure the box "Mirror Displays" is checked.


While in the Display settings you can also change the screen resolution to produce the optimum size for your displayed material.