A short (05:33) video that shows how to build a "stackup" that assigns GDSII layers their position and Z height; also shows how to select the region that you want to convert into a 3D model.
A short (mm:ss) video that shows how to export STEP or parasolids from the 3Di file created by Extract3D
A short video that shows how to import parasolids into SolidWorks.
An alternative stackup approach