I'm not an expert programmer but have some experience. This is what I've deduced after watching the YouTube video on the Waveform API and looking at the provided source files.
The DSP_API is written in C++ and are text files. They need to be compiled into executable binary files. Executable files need to be compiled for the particular microprocessor that they will be run on. For the Radio this is a 1 GHz ARM M8-hf (Neon) CPU. It is mentioned that a BeagleBone Black is a compatible hardware module and I imagine it can be used for development (compiling, etc.).
The DSP_API\DOCS\Documentation.txt file indicates the folder in the radio where the executable and config file must reside. But, it does not describe how to transfer the files to the radio.
The PC GUI is written in C#. It is intended to run on Windows PC and can be built using Visual Studio. The FlexLib code or dll's are not provided in the download and the current FlexLib.dll that I have (v22.214.171.124781) does not include definitions for the Waveform API. So, I get errors when trying to build the GUI.
These are not trivial tasks. We will have to wait for Flex to provide more information.