Depends on what type of C++ it is: if it is CLR code, then you can just add it as a reference and call it's methods as if they were C# - the framework will sort it all out for you.
If it is native C++ code, then you may be able to use it, but you will have to read up on P/INVOKE and will have to find out what functions the C++ code exposes.
You can add a C++ CLR project to your solution very easily: Right click the solution and select "Add...Existing Project..." then follow the dialogs from there.
The other alternative is to use it as is: you can execute it as a process and it may be possible to provide it with command line arguments to make it work without it's GUI (which I assume it has from your "push calculate" comment.
There is another option: rewrite the C++ code which does the work in C# (and ignore the GUI if you don't need it).
Sorry, but we can't give a definitive answer based on that little info!