The documentation for C++ is virtually non-existent. However, I managed to piece together a skeleton C++ Add-in that creates and displays a Task Pane in Excel. The solution contains 2 projects - the addin and an activex control. The projects are configured so that COM registration will be performed per-use when building. It was developed using the 64-bit version of Office 2013 and VS2015. The barest minimum was done to create and show the Task Pane so at best it is a starting point. You can download the project from OneDrive https://1drv.ms/u/s!AmnqrCFBv4nDghcZwI3yqeTaOcBR?e=psnZ44
Com Addins per Excel -
Task Pane shown in Excel -