Hey @GeonBit
You are looking for the MonoGameContentBuilder (MGCB.exe).
It should be located here:
%ProgramFiles(x86)%\MSBuild\MonoGame\v3.0\Tools
Please take a look at this post where I describe how to use it:
Also, when working with the MGCB I recommend taking a look at this:
http://www.monogame.net/documentation/?page=MGCB
Cheers! 