Where the treadmill doesn't have your weight then the calorie counter isn't accurate but that doesn't mean the number they give is useless. It still gives you a measurement of how hard your workout is. If you do a slow run for 60 minutes and it says 600 calories burned. The next day you do 30 minutes of fast interval training and it says 600 calories burned then you know it took the same effort to do both workouts.
The above treadmill calorie counter is good when the one on your machine is not accurate. I would say don't obsess about how many calories you've burned. It is interesting and you may be trying to lose weight so it can be motivational when it's a good high number. The number that is most interesting and accurate are the scales when you weigh yourself. This is the important number.
Try to weigh yourself weekly in the morning as soon as you awake at the same time and you'll tell how much weight you could be losing. A calorie counter will only tell you so much and the scales will tell you if you're succeeding in losing weight. We have written a page on how to lose weight on a treadmill and there are many workouts which can help as well.
We have written many weight loss on a treadmill articles that you'll find useful where we've gone through how to lose 10 pounds a month to 500 calorie workouts. These are all for the treadmill so you'll be able to get the maximum use out of your machine. It really has the ability to help you get fit and lose weight even if you walk on it.
The key to burning a lot of calories is to build up your workouts in time. You may not be able to last long at first but you build up the time you're walking or running over many weeks and you'll be able to run for an hour or so. You will start to notice great weight loss and you should approach your ideal, healthy weight. You will also start to feel good as the positive chemicals race around your brain.