Hello,
have you seen the example "Device Integration" that is provided within the set of examples in the Build Environment for Raspberry Pi 3? It is located in the subdirectory /Examples/DeviceIntegration and shows the basics about starting a certain action from the GUI and updating the GUI from the underlying system. It is intended to serve as template for your own developments.
You own C files have to be added to the makefile in order to compile them together with the GUI application and other software packages. There is no need (or possibility) to put C code into an Embedded Wizard Studio.
I hope this helps...
Best regards,
Manfred.