After installing Office Web Apps I tried to activate the Site Collection Feature, which gave the descriptive error: File not found. Thankfully Tonys did know what to do with this problem as described here.
The event viewer returns errors about the Alternate Access Mappings that hasn’t been configured yet and that users are accessing the site through unregistered URLs. To fix this add the URL that is indicated, and the activation of the Office Web Apps Site Collection Feature will go through smoothly.
Hope this helps someone!