When I looked at the guide you posted I did not see reference to the Build plugin menu option. However, we may be looking at side versions of the documentation. The Build Plugin Option was visible when using the Magic Leap 1 SDK since it used a custom OS. Magic Leap 2 runs Android, so plugins should be built using the standard Android tools and workflows.
Regarding the plugin not working as expected, this might be due to an issue with the plugin that is included in the plugin . Here is a link to a post I made about getting the plugin to work on Magic Leap 2 : Using the PDF Renderer from Paroxe