I have a DLL add-in for Excel that adds an API for another program I have. Add-in had an installer package and after install I can't find it in the Excel add-ins list. I have a suspicion that it is a 32-bit addin that may be causing the problem. Is there a way to check if the DLL is 32-bit? Even if it is 32-bit should it cause a problem to run on 64-bit Excel? Help!
You can refer to the question, How can I find out if a given program is 64 or 32 bit version?, for that. That will work for DLLs and libraries, too.
Yes. 32-bit programs cannot load a 64-bit library, and vise-versa. So if you have a 32-bit Add In that you need to use, then you need to install the 32-bit Excel.