Hardware Devices and MIDI Designer
MIDI Designer works with all CoreMIDI-enabled devices. We love the iConnect, iRig and Line6 MIDI Mobilizer II, and all others seem to work fine. Even the Alesis I/O dock works great but will not transmit sysex messages. There are some new entries that should work fine, too, like this product by Griffin.
However, this question is about getting to your computer without Wi-Fi. That's a more interesting question...
The iConnect works amazingly well with MIDI Designer 1.3.8. If you've got the funds, it's the best way to connect without Wifi.
Your other option is what I call the "iConnect Lite": use an iRig or an Line6 MIDI Mobilizer II. Then you will need some way to get MIDI back into the computer, so you'll use a USB MIDI device on that end. Something like this will work.
iPad Camera Connection Kit with Two-Sides USB Cable: NO!
You may not use an iPad Camera Connection Kit and a two-sided USB cable to connect to your computer. However, you can connect to class-compliant MIDI devices directly via the iPad Camera Connection Kit.
Note on Wired Connections
Wired connections are great, but ad-hoc wireless connections are quite amazing too... unless you have some massive amount of RF interference or you're James Zabiela and worried that someone will hack in and ruin the show. If you want to play drumpads on MIDI Designer, a wired connection is probably a good idea, too: the latency is definitely lower and, more importantly, more consistent.
We now support Bluetooth in MIDI Designer Pro 2.