Advanced tab on Model Options dialog.
You can set the height of the working plane (otherwise known as desk height) to define the z-position of the lighting sensor. The x-y coordinate is set by moving the lighting sensor marker(s) at zone level.
The Working plane height is used for EnergyPlus daylighting simulations only and is not used in Radiance daylighting where a separate value is defined on the Daylighting calculation options dialog.