Do NOT try the installment agreement in the TT program after 4/15 ... instead choose the Mail in a Check option and either make a payment of some kind now with the voucher ( cross out the balance due and write in what you are sending) OR wait for the IRS to bill you OR go online at www.irs.gov/payments to do an installment agreement there.
FYI ... since you already have an installment agreement for another tax year it may become null and void and a new installment agreement may not be possible ... if you read the installment agreement fine print it says you must not get behind in your taxes again ... if you do it voids the agreement.