First: You need the version of the Application Builder that matches the version of LabVIEW that you have. I your case 5.1, your executables will also need the run-time engine for the version of LabVIEW you have. To obtain the Application Builder you have to call National Instruments and it is free with the upgrade from 5.0 to 5.1. You should also know that the last update for LabVIEW 5.1 is 5.1.1 so if you do this update you need the appropriate app builder. If you have LabVIEW Professional Development System, the app builder is included.