Have a read through the MIDI tutorials included in the documentation, they're really helpful and will point you to [ctrlin] or a combination of [midiin] and [midiparse] which can work better if you're sending large amounts of controller data, due to a problem with [ctrlin] (Is this still a problem? Anyone?)

lh

very cool flanger!

Try this for the [ctlin]. it's best to "hardwire" the controls if you know what controller # and MIDI channel they're sending on. Or you can make them switchable with some [select] magic. The ctlins are always listening unless you disable them, so having only the ones you need is best.

here an augmented version of flanger :