The following assumes you want to calibrate a monitor on a Mac OS X stimulus computer.
If you have not installed the spider calibration software, do that first.
Quit Matlab if it is running.
Plug in the Spyder calibrator to a USB port on the stimulus computer. You may wish to use a USB extension cord if the stimulus monitor is far from the computer.
Open System Preferences, and Go to Displays. Go to "Arrangement" and move the menu bar to the stimulus monitor. The program Spyder Express 3 can only calibrate the main monitor with the menu bar.
Change the resolution on the stimulus monitor to something like 1280 x 1024. Spyder Express assumes a minimum high resolution (its window will not fit on an 800x600 display, for example).
Open the Spyder Express application. You may wish to search for the word "Spyder" to find the application.
Follow the on-screen instructions to calibrate the monitor. When it asks you to "Place Spyder here", hold the Spyder gently against the monitor during the whole calibration procedure. Keep it as still as possible.
At the conclusion of the calibration, open Systems Preferences and Displays, and look at the "Color" settings of the stimulus monitor. It should show that it is using the "Spyder Express3" configuration.
Move the menu bar back to the main monitor (usually the main monitor is not the stimulus monitor).
Change the resolution of the stimulus monitor back to the proper resolution (800x600, usually; 60Hz refresh rate for an LCD, and 100Hz for a CRT).