How to call Matlab functions like: rgb2gray() and imresize() in VSC++ ?

I have configured my Visual Studio 2012, adding matlab libraries and header files into it, but I dont know how to call functions, or I dont know which header file should I include in C++?

One more question: How can I convert an image into 4-bit image using Matlab function in VSC++? I am very new to Matlab.

Thanks in advance.