I originally had office 2013 and then inventor. I recently upgraded to office 2016, so the sequence of actions may have been the cause for the object library missing from the references window.
I figured it out though. The main issue I was having was determining the location and name of the dll file. For me, the object library was only missing from the references window while using vba editor in Inventor. When I open up excel and pull up its vba window, the object library was not missing from the list.
So I clicked on the Microsoft Office 16.0 Object Library from the excel vba references window and it shows the full file path. But it cuts off before the end. I had a nightmare with this and Microsoft help chat/calls.
The next morning I opened Visual Studio 2015 and opened a Visual Basic > Windows > Classic Desktop > Console Application. From the Solution Explorer, I right clicked References > Add Reference. A window came up with an extensive list of object libraries. I found the Microsoft Office 16.0 Object Library that I was looking for, and when I put the cursor over the listed object library, it displayed the full path and file name uncut and uncensored.
Finally, I went back into the inventor vba window > Tools > References. I then clicked browse in the new window and found my dll file. It was named Office once it was in the References window. I closed the references window and reopened it to find the Microsoft Office 16.0 Object Library just as it should be. All of this just so I can create File Dialogs!
Anyways, I can't imagine I am the only one in the world who had or is having this issue with an object library. For Office 2016, here is the full file path, which I imagine will be the same for any user because I believe it is a default directory:
"C:\Program Files\Microsoft Office\Root\VFS\ProgramFilesCommonX86\Microsoft Shared\OFFICE16\MSO.dll"
This is for the Microsoft Office 16.0 Object Library.
If anyone is having a similar issue with an object library and they know it is on their computer somewhere, employ the aforementioned method using Visual Studio 2015; Community Edition should work fine.
I can't stress how much of a nightmare this was for me and how much time I spent searching for this. Now I know and I want to make sure others can know too.
Good Luck to all and Happy New Year.