>> Select one of the active tabs - Path to Gcode, Area, Engraving, DXF points, Orientation, Offset, Lathe or Tools library.
Just do what is says

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Gcodetools has a lot of functions so it needs a radio buttons or something like that to select the certain function. So we have decided to use the name of the currently opened tab at it's form.
So if you want export Path to Gcode, open path to Gcode tab and Press Apply.
If you need Engraving open Engraving Tab and press apply.
Such way you can run several functions from one extensions window. And when you press apply from "non functional" tabs such Preferences or options Gcodetools raises an exception asking to choose proper tab.
PS in windows you have to put extension's files into C:\Program Files (x86)\Inkscape\share\extensions\ (probably you'll need admin wrights to do it) or in your own user settings directory, something like C:\Documents and settings\user\application data\inkscape...
On ubuntu it's /usr/share/inkscape/extensions/ and /home/nick/.config/inkscape/extensions/ .