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.
Another thing to keep in mind is interest. Left as is, the loan will keep accruing monthly interest. If your loan is not going to be accruing interest, use the promo rate option on the debt details page and set the percentage rate to zero and have it expire the same month that the deferment ends
*Tip* If you have a deferred loan and you don't want the payment to be reallocated, create a negative amount debt snowflake for each month you want to skip a payment.