I was approved for a 120 day payment plan by the IRS. I selected the "mail a check" option when e-filing my taxes. Where do I go to make payments on what I owe?

When you set up the installment agreement were you not asked how you were going to pay the amount due?
It should be paid with automatic payments from your checking account or check, money order or debit/credit card.

120-Day IRS Payment Extension

The IRS has an official 120-day payment agreement. As long as you owe less than $100k in combined tax, interest, and penalties, you can apply for this online. Use the Online Payment Agreement application—it’s the same application you use to apply for an installment plan.

There is no fee for setting up a 120-day IRS payment extension. However, your unpaid taxes will continue to accrue interest during this time. In most cases, you pay less penalties and interest with this plan than someone who chooses an installment agreement.

If you already set up the plan then you can pay online here :  https://www.irs.gov/payments

I owe around $55K to the IRS and plan on filing a 120 day extension to be paid off within that time frame.

Will the IRS file a tax lien on my credit or home?