I faced the same problem and could not come to a conclusion. Tried to create really everything that was possible in solution studio, but could not access that stuff with REST methods.
Only thing that works is create SOAP services and call them to do things, which is not really convenient for mobile apps.
SAP has some nice articles that appear great for the first time
External Integration of Custom Solutions in SAP Business ByDesign / SAP Sales OnDemand
but as nothing is really connected and explained in depth, I could not come to a good solution.
If you find out something, please share...