How do I include the deferred payments in my plan?
Undebt.it supports deferred due dates, so just add it in like any other debt account and choose the actual due date. Check out the Debt Snowball Table and you'll notice that there won't be any payments due on your student loan until the month when the payments are scheduled to kick in. One thing to remember is that the payments on the deferred loan will be automatically reallocated to other debt accounts until the payment is due.
*Tip* If you have a deferred loan and you don't want the payment to be reallocated, create a negative amount debt snowball for each month you want to skip a payment.