If you have not yet filed your tax return, click on Wages & Income then I'll Choose What I Work on then click on Start beside Dividends on 1099-DIV.
If you have already filed your return and it has been accepted, you will have to complete an amended tax form 1040X to add the 1099-DIV. Wait until you receive your refund from your original return before completing, printing, signing and mailing the amended form.
Here are instructions on how to amend a return: https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
