At http://ezquake.sourceforge.net/docs/?setup under "Startup - Windows", it says
If you don't have Start menu shortcuts, go to your Quake directory (the directory where you have installed or extracted ezQuake client) and run ezstart.exe. This will start the OpenGL version of the client.
But there is actually no ezstart.exe in ezquake_win32_1.8.3.zip. There is only an ezquake.exe and ezquake-gl.exe.