首先这是一个it was...that强调句,强调的是主语,去掉之后,句子为
The training that he had received in that well-known trans national company made him such a good sales manager.
the training 是主语,其后跟了个that引导的定语从句,the training在定语从句中作receive的宾语,句子主干是the training made him such a good sales manager.
意思是“他在那家有名的跨国公司里所接受的训练,使他成为了如此优秀的销售经理”