XML is case sensitive, but that doesn't appear to be the problem. Try:

DECLARE @XML AS XML, @hDoc AS INT, @SQL NVARCHAR (MAX)

SELECT @XML = XMLData

FROM #XMLwithOpenXML

EXEC sp_xml_preparedocument @hDoc OUTPUT, @XML

SELECT AccountNumber,ShortName

FROM OPENXML(@hDoc,'//Account')

WITH

(

AccountNumber [varchar](50) '@AccountNumber',

ShortName [varchar](100) '@ShortName'

)