The Nordic Track ski machine is just that - a piece of aerobic equipment that mimics the cross-country ski motion of the lower body while providing pulley-motion (cables with handles on a flywheel) for the upper body, to recruit more muscles that add to cardio output.
I did Nordic Track fairly consistently for about a year back in the early '90's (my early days of getting into exercising consistently), and although I did get a fairly good cardio workout, in retrospect I don't think it was that that great, especially if that is the only cardio machine you're using. The number of muscles recruited is fairly limited, the range of motion is limited, and thus the motion is so repetitive that 10 minutes feels like an hour on the brain AND the body. If it's free, I'd say take it, and do it when you want a brain-dead workout, but don't rely on it too much.
A-Jock