The namespace declaration must be part of the select statement. Try this:
DECLARE @BrandData BIT
; WITH XMLNAMESPACES('http://MCH.com/IF002/SD/MCGG/MCH' as ns0)
SELECT @BrandData = @xmldoc.exist('//ns0:Brand_In/MC/MC1')
IF @BrandData=0 BEGIN RAISERROR('MC1 not available in document',16,1); RETURN ; END