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'
)