I am a reservist who travels 108 miles for drill; where do I put the mileage in?

Go to Deductions & Credits, Employment Expenses, Job-Related Expenses. 

Go through the screens until you get to Did your Job-related Expenses Fall Into Any of These Categories? 

Enter the amount that you've put in for your mileage (.54 for 2016) under Military Reservist Travel Expenses.