From the main menu, select On Demand, arrow right until you reach the Networks category, go down to the Networks A-Z row and select the "A through C" tile. It will be one of the first options listed there.
It should also appear in the "Recently Viewed" row if you have watched a program on that network recently.
Alternately, you could spell out "Acorn" on your remote to search for the Channel. That translates to the numbers 22676.