I was just trying to use Robotium in an Android JUnit Test, but the Testing always fails with an error:
java.lang.NoClassDefFoundError: com.jayway.android.robotium.s
I had the same problem today. I solved by importing the jar into the libs directory of the test project and then adding it to the Build Path Libraries:
TestProject Properties -> Java Build Path -> Libreries -> Add JARs... -> TestProject/libs/robotium-solo-3.1.jar