If you're using MQ 7.0 or later, then MQException is in the jar file com.ibm.mq.jmqi.jar. First, can you make sure that this jar is present in DEVTEST_HOME/lib/shared/, along with the rest of the MQ client jars?
Generally, NoClassDefFoundError means the class file was actually found on your classpath, but an error occured initializing the class. A lot of times this is because another class is missing, but the NoClassDefFoundError itself isn't going to have any details. Whatever went wrong with initializing the MQException class has already occurred.
Can you check your workstation log? Find where that NoClassDefFoundError is, and then look above that in the log for previous errors.