java keyboard class error

i have win 2000 pro, i have copied the file cs1.jar into my 2 ext folders as it stated i should do…i have the path and setclasspath in my environment variables…i can compile and run no keyboard class projects …and i can compile but not run anywith the keyboard class..i get this error:
Enter your midterm grade: Exception in thread “main” java.lang.NoClassDefFoundE
rror: cs1/Keyboard
at averageif.main(averageif.java:22)
what can be the problem?
thanks
sr