I blogged regarding a particular error that you will received from not entering the correct Action during the WCF-SAP send port configuration in this post. but I didn't tell you what to put in there.
What should be in there is something like: http://Microsoft.LobServices.Sap/2007/03/Types/Rfc/BAPI_NAME_YOU_WANT_TO_CALL. Replace the last part with the appropriate value.
The easiest way I found to do this is by creating a WCF console application to consume an Adapter Reference to SAP and basically pick the BAPIs you are interested in and after it created the sapBinding.cs file, just open it and browse for the BAPI that you wish to map to WCF-SAP port, copy and paste the URI string that will look similar to the one above into the Action textbox of the WCF-SAP port setting.
If anyone know of a better way, please let me know.