binary trees in C++ - how to access the variables in the functions provided

I am currently trying to use the functions provided in:

Binary Trees in C++ - Cprogramming.com

which is the 18th Lesson of the C++ Tutorial. The code is provided to define the functions of a binary tree, but I am having trouble understanding how to use send arguments to these functions (am I passing an address, pointer,...)

Is there any code which uses these definitions and declarations?

Thanks,

David