Copy all .C and .H files (except RESOURCE.H) from your original project to the new MFC project. Also copy any other files specific to your C-language project — but don't copy your old project's .MAK file or its .RC file. The MFC project already has its own versions of the files you don't copy. Besides the .MAK and .RC files, you can skip copying such Visual C++ housekeeping files as the .APS, .VCP, .WSP, or .VCW files.
For SHOWDIB, you'll copy DIB.C, DLGOPEN.C, DRAWDIB.C, PRINT.C, and SHOWDIB.C. You'll also copy SHOWDIB.H and *.BMP. If you're doing the 16-bit SHOWDIB sample, also copy DLGOPENA.*.
Rename all .C files in the MFC project directory to .CPP. Then add the renamed files to the project — that is, besides copying the files into the project directory, you must formally let the project know that the new files now belong to it.
Tip To formally add files to your project in Visual C++ 2.0, use the Project Files command. In earlier versions, use the Project Edit command. See your C++ documentation.