I’ve done some mucking around on Google but I still can’t find what project this would be in Visual Studio. I want to create a GUI add in for Excel, for example a dock window on a ribbon with a windows form. What project would this be in Visual Studio? I have tried Excel add in but I don’t know if this has the GUI component I’m looking for.
Microsoft offers you “Visual Studio Tools for Office” (VSTO) for this purpose, see here for more information.
But if you want a more lightweight solution, and you need it only for Excel, I suggest you have a look at Excel DNA
This is freeware, mature, works even with the Express editions of Visual Studio and different from VSTO you can combine every supported .NET version with every Excel version and every Visual Studio version you like. I have created some Excel addins with Excel DNA on my own, and I love it.