I followed these tips. Using iPhone Explorer, Kindle can be found in a folder called com.amazon.Lassen. That folder contains folders labelled Documents, Kindle app, and Library. I used the path com.amazon.lassen/documents/eBooks and copied my MOBI files there.
Assuming that was the correct path; how do I open the MOBI file on my iPad? I can't figure out how to access the files from the device.