iDevices tend to be locked down a lot, I believe there is no straightforward way of connecting to them. Try using the
iOS reader applications plugin and beware that it doesn't work with iBooks on OSX 10.9 (Mavericks).
A good general resource for iDevices used with calibre is
calibre manual FAQ -- iDevices or the following MR thread:
Calibre + Apple iDevices: Start here