How do you set up Excel Services for SharePoint 2010?
I’ve installed the Office Web Apps (wcsetup.exe) and created a new Excel service in central administration.
But I’m now stuck on what to do next - do I need to turn it on in a document library?
You need to enable the "SharePoint Server Enterprise Site Collection features" Site Collection feature.
And Office Web Apps is not Excel Services. In Office Web Apps (is OWA the correct acroynom?) you have the Excel editor/browser which is not the same as Excel Services.
You first have to add your site collection url to the list of trusted excel services locations. After that you should have the option to open in web in any document libraries in that site collection.