On most keyboards it is the button next to the F12 key. It should read "Prtsc/sys req." If you have that button it should work, assuming your keyboard is in good condition.
To test it simply press the button at any time, and then open a program such as paint. Click on "edit" in paint and then click on "paste." This should paste an image of what was on your screen at the time you pressed the "PrtSc/Sys Req" button.
As far as I know this is the only way to capture a screen shot.

Let me know if this works or not. We can look into it further if it does not.

Thanks,
NDN