Deductions & credits

For me the "gotcha" here is that you can't just enter a number like $300 in that field, you have to enter at least that much in itemized donations. So under Federal > Deductions & Credits > Charitable Donations > Start Or the Edit/Add button at the top if you're already entered some. Once you're over the the $300 threshold (or however much you've donated) you can enter $300 (or however much) for donations like you would normally.