We are aware of a bug in the Freedrum apps for smaller screen devices. Ex. iPhone 5S.
In the image attached you can see that next to the sticks checkbox is a line. That is where it should have an additional checkbox and say foot. Though you can’t see it you can press next to the sticks checkbox to the left and select foot. Then it will set your sensor to foot.
We are aware of the issue and will add it to the next update of the app.