how can i find out were my o2 senser plugs in?

i have a peugeot 307 hd red i xsi 2.0 diesal and im gettin a anti pollution fault. ive had a garage put a snap on machine on it and its come up with o2 senser before cat not workin. so he had a look and the senser aint working because it aint got no were to plug in to .please help me if ya can.54 reg 40.000 miles