CBS-HD on 148 is the LA CBS-DT, exactly what you receive OTA in LA.
CBS-HD on 61.5 is the NY CBS-DT, also what you'd receive OTA in NY.
I haven't received the letter you got, but it probably was just a form letter trying to communicate that (if you didn't already have it) you would need a separate dish pointing to either 148 or 61.5 to receive CBS-DT via Dish, unrelated to your 110/119 Dish 500.
Of course if you already receive LA CBS-DT via OTA then the 148 Dish-provided programming is of no value. If you point to 61.5 for your Dish-provided HD then you could pick up NY CBS-DT three hours before you get OTA LA CBS-DT.
While the channel is there, it doesn't show up on your Guide unless they activate it for you. Depending on certain requirements, you're either (1) not entitled to it, (2) entitled but must pay $1.50 for it, or (3) entitled to receive it for free. Once activated, it will appear in your Guide.