Right click and enter Full Screen Mode to test please.

 

LMB - Drag to Select Units, Place Buildings, Select GUI

RMB - Move Units, Cancel Building Placement

 

GUI Options : 

When Worker Units are Selected : Building - Allows you to place the Buildling object

When Building is Selected : Worker - Allows you to build worker units

When Building is Selected : Militia - Allows you to study the militia technology

When Building is Selected : Militia Unit - Allows you to build militia after you have researched the technology