You should be getting channels 300-314 (all the HBO channels and all the Cinemax channels, with the exception of HBO HD). I think the only requirement to get the free preview is to be subscribed to something, not sure if there is a minimum.
Another thought comes to mind. Do you have any favorites that you've set up, and perhaps you are looking at that guide? If so, just press guide repeatedly, and it should cycle between all channels, all subscribed channels, and any favorites you've set up.
Yet another possibility, if you have a Dishplayer 7100/7200, I don't think they were able to get the last free preview, so they may not be working now either.
Let us know what model Dish receiver you have and perhaps we can help more...