How do I add the days worked abroad for an international organization? US citizen, working mainly from US, not subject to self-employment tax on income earned abroad.

You will enter this in the Foreign Earned Income and Exclusion section of TurboTax under Wages and Income, Less Common Income (unless you have a W-2). To exclude some or all of the income, answer the series of questions presented in the Foreign Earned Income and Exclusion section.  

If your organization issues Form W-2 (we're aware most don't when you're working abroad) you will enter it and then check the box provided on the "Less check for uncommon situations" page "Employed by a foreign government or international organization".  

If you are entering via W-2 screens, you will have an opportunity to indicate the amount of the wages subject to self-employment tax as $0.