Your airport express is a G wireless model which apple only supports in v5 utility.. you need a Mac running earlier OS or even windows.. any version windows can still run v5 utility.. because apple hasn't made a new one for it.
Otherwise you need to google.. Install airport utility v5 yosemite (it is a bit harder in El Capo but it works) This is tricky setup..
Apple basically wants you to buy a new express.