Start-Up Expenses are reported in aggregate - one amount equal to the total of all expenses incurred. For active business activities, these costs are entered either under Assets/Depreciation or under Business Expenses depending...
As long as your start-up expenses are less than $5000, you can add them as Business Expenses. Continue past the expense categories (or choose Other Miscellaneous Expenses) to the page titled "Enter Business Expenses Not Yet Reported" and enter the description & amount (see screenshots below - click to enlarge).
Per IRS Pub 535 Business Start-Up and Organizational Costs: "Business start-up and organizational costs are generally capital expenditures. However, you can elect to deduct up to $5,000 of business start-up and $5,000 of organizational costs paid or incurred after October 22, 2004. The $5,000 deduction is reduced by the amount your total start-up or organizational costs exceed $50,000. Any remaining costs must be amortized."
If you have more than $5000 in start-up costs, the remainder is entered under Assets/Depreciation as a capital asset for amortization (TurboTax provides this category for you).
If you have Rental Property (passive business) start up costs must be capitalized, subject to depreciation. Enter these costs under Rental Assets/Depreciation as "improvements." See this discussion for more: Start-Up Expenses for Rental Activity