Did it work before and your having a problem now, OR, this is the first time you've tried it and it doesn't work??
The model of STB your using is also helpful, my STB crashes when I try On Demand, likely because its a 10 year old DCT2244 with limited memory.
If your existing customer, you do NOT get On Demand automatically, you have to call COMCAST and have your package changed to a package with On Demand. If you had the basic expanded package a.k.a. the Standard Service, its the exact same price, why they convert everything automatically for you, except withhold On Demand, I don't know. I've been told your old package is grandfathered, which is laughable, but thats what I was told, existing customers have call and have their package changed to get On Demand, even if they are being charged the same price for the package with On Demand.
My DCT2244 crashed and rebooted, with all guide data lost, every time I tried On Demand. I bypassed my home cable network and connected the DCT2244 directly to my cable drop coming into the home, it did the same thing. Traded the DCT2244 in for a DCT700, On Demand works.
Its possible my DCT2244 was defective, but being built in 2000 it likely has the weakest capability and most limited memory of COMCAST's equipment.
The DCT2244 had a decal on the bottom of the box that listed the model number as DCT2244.
Pretty sure its a part of the larger group of DCT2000 boxes, the appearance is exactly the same as DCT2000 and the DCT2000 manual appears to apply, perhaps there are others models other than DCT2244, within the family of DCT2000.