Thanks for the question. Along with Audeonic and Secret Base Design, we are co-authors of Music IO. It works perfectly well with all editions of MIDI Designer to provide bidirectional MIDI over USB to Windows (currently in Beta) and OSX.
The MIDI-only version is currently free. In addition, Music IO provides audio over USB: both of these uses are awesome and bring iOS into the professional music-making workflow.

From the Music IO website:
Music IO is a joint effort by three of the most respected iOS music app development teams. Confusion Studios makes MIDI Designer Pro, the most-respected MIDI controller platform for iOS. MIDI Designer is adored by users, loved by Music Industry press, and trusted on stages and in studios around the World. Audeonic Apps makes the pioneering MidiBridge app, an essential tool for routing and filtering MIDI messages on iOS; they also provide the MidiBus software library, which powers dozens of successful music apps (including Music IO). The Secret Base Design team has built a range of apps providing high-speed MIDI and audio connections over Bluetooth LE and WiFi, and also develops innovative signal processing apps.