Hi -
This issue is due to a known bug which will be fixed in the coming weeks.
In the meantime, there is a workaround to get the Find Hot Spot tool to become enabled. Use the 'Share Layer' button to share a layer to ArcGIS Online. This creates a hosted feature service. Once the layer has been shared successfully (the layer will have a green check mark next to it when it has published successfully), sign out of Maps for Office and sign back in. The Find Hot Spot button should now be enabled.
Thanks,
Scott