On my machine with TestStand 2013, the Parallel UUT dialog is in a file called paralleluutdlg.c at the following path: C:\Program Files (x86)\National Instruments\TestStand 2013\Components\Models\TestStandModels. Your path may differ slightly if you're on a 32-bit machine or if you have a different version of TestStand.
What type of change are you wanting to make to the dialog?
You can also just override the dialog with code in a sequence using callbacks, replacing it with whatever you want. See the example: