I owe $ 5,662 for my Federal return. How do I set up an installment plan? I already have an installment agreemnt. Can I add it to that, or do I need to wait to filew the

You will want to contact the IRS as soon as possible to have the new balance due included because a new balance will automatically default your current agreement (It is one of the terms and conditions.)

You can call them at 1-800-829-7650, although there will be long wait times since it is peak tax season. You can go to local office (http://www.irs.gov/uac/Contact-Your-Local-IRS-Office-1), but they also have long wait times. If you submit another IRS Form 9465, you can include all of the tax years that you owe with the request to have the new tax year included. It is important to have the new tax year included because collection will start on the new tax year, if it is not added to the agreement. You will know when your return is processed because you will receive a tax bill.

If you owe because of withholding you can use TurboTax to change your W-4, so that you do not owe again next year.  

Here is some more information from www.irs.gov: http://www.irs.gov/Individuals/Payment-Plans-Installment-Agreements