May I use my IRA account to pay off "parent plus" loans?

Since you are over 59 1/2, you can withdraw money from your IRA for whatever purpose you like without the 10% penalty. You'll still need to pay income tax on the amount though, as the withdrawal will count as income. So be careful that you don't take out so much that you can't afford to pay the tax (and/or get a penalty for underpayment) when you file. It may push you into a higher tax bracket. Not to mention the opportunity cost of future gains from this money.

There generally aren't ways to avoid paying the tax on Traditional IRA withdrawals. There are cases for avoiding the penalty, but paying student loans is not one of them. Paying qualified education expenses is, though, so if you had paid the tuition from the IRA instead of borrowing, you would have avoided the penalty (but not the tax) before age 59 1/2.

If your IRA is a Roth IRA then you can also take out the money for whatever you like without penalty, and there's no tax consequence. The only consequence is again the opportunity cost of lost growth.

If you are still employed, you probably can't tap your 401(k), but check with your employer to see if you an do an "in-service" withdrawal, either in general or specifically to pay off student loans.