You are allowed yes, but just don't go too crazy. If you used it before, you should be ok, just don't do too much. Learn to know when your body's being pushed too much. If you weren't really doing any exercise before, CD recommend not taking it up in the first few of weeks. Just let your body adjust to the diet and slowly work it in. But I think you should be fine, as long as you don't do too much.
As dappy said, resistance may be better. That will work on toning up muscles, but also it carries on burning calories for longer than cardio.
Do you have the cross trainer at home or are you going to the gym for it?
I really should start putting all this theory into practice more!