I paid some mortgage interest to a lender, but less than the $600 threshold to generate a 1098, how do I report that in tax return?

Pretend you have it in your hands. The IRS knows about the 1098 rules for issuing a form.

Go to federal, deductions and credits, select Mortgage Interest, enter the lender, select none of the above as if you have a 1098, enter the information in the boxes.

You will see a What if I don't have my 1098? link on the screen where you fill in the information.