Where to enter Google Maps API Key

I have the same issue / question. did this ever get solved?

You will need an API key from here:
https://developers.google.com/maps/documentation/javascript/get-api-key

You will need to edit some of the source files until the developer updates this plugin.

in the plugin directory:
register_fields.php line70
change https://maps.googleapis.com/maps/api/js?&v=3.exp&libraries=places
to
change https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&v=3.exp&libraries=places

in the plugin -> view directory:
single-ecwmv-events.php line 56
change https://maps.googleapis.com/maps/api/js
to
https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY

ecwmv_events.php line 222
change https://maps.googleapis.com/maps/api/js
to
https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY

That worked for me, although I have not fully tested yet. I will update this post if I discover anything else.