Not able to use Custom field to display a map

Hi

I'd like to use the Google Maps plugin to display a listing location as a map on the listing page.

I created a CustomPress custom field called showonmap which I then added in Settings > Google Maps > Custom Fields > My posts have an address field name = showonmap

I thought this would be how it works, but it doesnt appear to show anything on the listing page.

Could anyone help?