How do I create a contract for eBooks on iBooks?

I've successfully submitted a free eBook on iBooks... almost.

Recently, I got this message:

"No Contract. You do not have a contract in effect for this territory. Please work with your iBooks representative to enter into a new contract if you wish to sell your books in the territory in question."

How/Where do I set up this contract? Why is this necessary if my book is free?