How to add #include and #define to a Windows Form Application

I created a simple Windows Form Application using Visual Studio 2012. I have successfully added simple functions that I can call from, for example, a button event.

Now I want to add more functionality and more complicated functions. I have used these functions in other regular C programs. But in order to use them I need to #include a .h file. The .h file also contains #define statements and other #includes (e.g. #include<stdio>.

I can't for the life of me find anywhere in the project that will let me add a .h or #include or #define???  The three files (form.cs, program.cs and App.config give errors whenever I try to add one of these.

I have spent many frustrating hours on this. What am I missing????????