Place the files (*.py and *.inx) into the Inkscape extensions folder. This is probably similar to one of these:
Just drag and drop the *.3d file from your file manager to the Inkscape canvas. You might also use the "File > Import... (Ctrl+I)" dialog from the menu. For the ability to import with extended view (View = Extend) the file must either be already extended, or the Survex installation folder must be in your PATH environment variable so that the "extend" (or extend.exe) binary can be found.