Where do I enter the NJ Anchor property tax rebate amount on my Federal return?

You have to deduct that amount from your property taxes inputted on the federal screens if you itemize your deductions. 

On NJ return, 

1. click on Taxes and Credits
2. Continue to the screen Eligibility - Property Tax Deduction or Credit
3. Answer the questions and continue 
4. Enter the full amount of property taxes paid

For more details check LenaH response