I tried to to download 'Export to Excel' from the App Store too and got the following message:
We have a version of 'Export to Excel' working on Tridion 8.5 at my current client. Unfortunately this isn't my code, so I can't just share it, so I suggest that you contact SDL Support to get a version from the App Store.
However, although the 'Export to Excel' button was available on the Search Results screen, nothing happened when I clicked on it.
I then saved the search results into a Virtual Search Folder, but the 'Export to Excel' tool gave an error when I clicked on it.
I then created a bundle and added all of the items from the Virtual Folder into the Bundle and the 'Export to Excel' functionality worked and gave me a CSV file.
You could probably miss out the middle step here (creating the Virtual Search Folder) and just add the search results to a Bundle and then use 'Export to Excel'.
Hope this helps!