Aperture and pickbox are often confused. The Aperture is only active when ZWCAD is expecting the user to pick a point, and only when OSNAP is enabled. By default, the Aperture is not displayed, therefore moving the slider would have no visual affect.
The Aperture slider is on the Options > Drafting tab, whereas Pickbox slider is on the Options > Selection tab.
To test, turn the Aperture display on (APBOX = 1) then with OSMODE something other than 0 enter a command that expects the user to specify a point, ie LINE command.
Depends on the graphics configuration, the effect of Aperture/Pickbox may be displayed as like the Square or Rectangular, usually, it should be displayed as square.