The white connector at the end of that item in the link goes straight into the back of the RCD300 where you'll find the majority of wires leading to not sure on what its actually called but you'll know what i mean when you pull the RCD300 out.
My iPhone/iPod kit works in exactly the same way except i had to connect the earth from iPod setup to the earth of the bunch of wires mentioned above.
You shouldn't need to do this i don;t think and you shouldn't need any adaptors i am aware of just 'Plug and play'