So I was testing things in Maple 17 and Excel 2010 just because that's the setup I had left over from the last time I used the Excel add-in. I made some progress (got rid of the error I was getting) but ran into other problems (different errors).
Actually I'm using Maple 2015 and Excel 2016. Probably the multiple installations overwrote something, or the upgrade from Windows 7 to Windows 10 did.
I fixed the problem by completely uninstalling Maple 17 and Maple 2015, then deleting the add-in from the Excel add-in manager and deleting the Maple custom toolbar in Excel (right click on one of the Maple icons in Excel to get the delete option). Then I reinstalled Maple 2015, and installed the 32-bit add-in in both Excel 2010 and Excel 2016. No modification to the registry. So someone starting from a new installation shouldn't have problems, as long as they install the correct bit length version of the add-in, to match the bit length version of the Excel they're using. Some other messages I've seen suggest that upgrading Excel may cause problems, and certainly upgrading to a new Windows version may too.
Now the Maple help in Excel works, and the formula '=maple("x + x")' in a cell returns '2*x'. The Maple Function Wizard/Browser in Excel doesn't do anything - the browser opens but never populates with any function names.
From the Maple Help Browser in Excel, there is a button Using Maple in Excel. This is supposed to open a Windows help file which describes how to use Maple in Excel and how to use the toolbar icons. It uses a format no longer supported by Windows. Microsoft has released standalone executables to view these old help files, but has yet to release one for Windows 10. If you click the Using Maple in Excel button and get taken to a Windows error web page like this, it's because your Windows doesn't support the old help files and you need to install the standalone executable.