In Arcmap, go to Geoprocessing on the top menu bar, and then choose Geoprocessing options. Down where it says Script Tool Editor/Debugger, in the slot that says Editor, put in the full path to idle.pyw. On my machine, it is "C:\Python27\ArcGIS10.1\Lib\idlelib\idle.pyw" It will probably be similar on yours.
good luck
Mike