Try this: click on the Federal Taxes tab, then Deductions & Credits, then scroll down to Estimates & Other Taxes Paid and select Sales Taxes.
Best to click on the Easy Guide button when you're prompted, as that will walk you step by step thru the process. You'll be asked some questions, you'll enter the sales tax rate for your state, and then you'll come to the "major purchases" question. Just continue to follow the prompts to make your entries.