Search for 1098-E and select the Jump to link at the top of the search results. Answer Yes to the question Did you get a 1098-E for student loan interest? If you land on the Here's your 1098-E info (or Here's what you paid in student loan interest) screen, you can either edit an existing payment or add a new one.
If both your names are on the Student Loan, you can claim interest you paid. As a co-signer, you are legally obligated to pay the loan, so you are allowed to claim the interest.