To use the source code in Simplicity Studio, first launch the Silabs IDE.

Then select File -> Import... and choose Simplicity Studio\MCU Project.

Browse to the folder you unzipped the source code and select 
EFM32G_sensor_Puck.slsproj from the SimplicityStudio directory.

Click next and choose the following options:
Kit: None
Part: EFM32G210F128
SDK: EFM32 SDK
Toolchain: GNU ARM
Build Artifact: Executable

Then click next and finally Finish.