How can a person break into their trunk of a 1996 Olds cierra with the keys locked in it?

get into the passenger part of the car and then remove the rear seats or call the AA

a good old fashioned crow bar

Easiest way is to call a locksmith to get you in. Methods of "breaking in" to vehicles will not be given here.

Check if there is a trunk release lever on the left side of the drivers seat by the gas door release lever.

You can't. You'll need to go to the Stealership, with your vehicle registration and ID, and have them order you one from Germany. Takes about a week and you then give them around $180.