200union,
At this time there is not a way to import MP3's or AIFF files into Capture for iPad. Capture for iPad was designed to work along with an audio interface to allow you to have a mobile recording rig and then be able to bring this audio back to Studio One and allow you to use your audio in a more powerful environment.
Their is a guy whom has figured out a work around and he has shared a video on how to get audio into Capture for iPad, while this feature is being worked on for future releases.