Using Gym Equipment for Weight Loss

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I have been going to the gym every other day for about a month and a half to lose weight, and I am confused about the best way to use the machines (elliptical, stationary bike, and glider/stair climber). I started off choosing the weight-loss option on the elliptical trainer, entered my weight and age, and it gave me a target heart rate of 121. Does that mean that my optimal weight loss workout would be to do as much as I can, at as high an intensity as I can, while maintaining that heart rate? It's very difficult for me to keep my heart rate that low because the workout is boring and feels way too easy, even with the machine on a high intensity level. I guess the bigger question is, is there really some optimum heart rate for weight loss, or is it all about burning as many calories as possible?

Also, how reliable are the machine's electronic calorie counters, considering that I enter my weight and consistently use the sensors to track my heart rate?