Click Start->All Programs->Research In Motion->BlackBerry JDE 4.2.1->JDE.
Click Edit->Preferences.
Click the Simulator tab and perform these steps:
Click the MDS Simulator Tab. Make sure that the MDS Simulator directory location is pointing to the v4.1.4 MDS directory you installed. For example:
C:\Program Files\Research In Motion\BlackBerry Email and MDS Services Simulators 4.1.4\MDS
Click OK.
You can leave the JDE running, because you may need it later on.