CMYK is an abbreviation of Cyan, Magenta, Yellow, and blacK, a color model commonly used in printing. Most computer graphics, including charts and figures created with Excel, use the RGB (red, green, blue) color model. To convert a graphic from one model to another, use of a color management system is generally required. A color management system ensures that colors are conserved between different outputs (e.g., screen and print).
If you need to generate a CMYK image from an Excel graph, you can enable color management in the XL Toolbox. You will need to have color profiles installed on your system. If you don’t know much about color profiles, it is probably not a good idea to try and make use of them. If you are a color management expert, you will certainly know that Excel itself of course is unaware of any kind of color management. The Toolbox can only assume a standard RGB color profile to do the conversion to CMYK.
If, for some reason, you absolutely must generate a CMYK image file, but have no color profiles installed on your system, the Toolbox can perform a very basic RGB→CMYK conversion for you. Be advised though that the results may be unpredictable. Even if the resulting CMYK file looks good on your own screen, it may look entirely different if someone else displays or prints it using a different hardware and software setup.