This letter, CP 523 is a notification to you that the IRS intends to terminate your installment agreement in 30 days. You have the right to request an appeal if you do not agree by following the instructions in the notice.
But you are asking what your balance due is. If you have not been using he Electronic funds Transfer Program with the IRS, the only way you will know, is to contact them by phone.
you can contact them at(NNN) NNN-NNNN select option one and option 4. They are open until 10pm Eastern Time. It is now 5:40 Eastern Time. So, there is still time to contact them. You can expect to wait on line for several minutes; just be patient, they will come on to help you.
If you get no results from that, then you can go to the nearest Tax assistance center, and they can look it up for you while you wait. You can even make payments there.
You can find the nearest Tax Assistance Center here: (scroll to the map) http://www.irs.gov/localcontacts/index.html