It can be accessed after finishing Level M-5.
At first this level is named F-1, but soon it is renamed as The River, part 1 when the child renames The tunnel after beating The River, part 5 (technically Level F-5)
Make use of the ramps and don't jump too far away. This can cause fatality. But in some spaces there aren't any ramps, so please pay attention. Be careful and you will be able to pass the level easily.
Overall, this isn't such a difficult level.