[vtkusers] How to set "$VTK_DATA_ROOT"?

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Hello,
       I always double click the file of its file extension is tcl , for example, vtk\Examples\GUI\Tcl\ProbingWithPlaneWidget.tcl, but always show the message: file not found.  This file include "$VTK_DATA_ROOT/Data/combq.bin" , but in fact the path of file is:"E:/Data/combq.bin".  The question is how to set the VTK_DATA_ROOT ?

Thank you!