You'll enter these by clicking on the Federal Taxes tab, then Deductions & Credits, then scroll down to Employment Expenses, then select Job Expenses for W-2 Income.
Then you'll follow the prompts thru the various job expense screens, answering the questions -- and you'll ignore the vehicle expense screen -- until you come to the screen that says Any Special Situations? Note that Armed Forces Reservist is listed, so you'll check "yes."
Then the very next screen will show you boxes to enter amounts for these special situations, and the very first box is for Military Reservist Travel Expenses. Since the federal standard mileage rate is .54/mile, you'll multiply your reservist miles x .54 and enter the total in this box. If you have other reservist travel expenses, you'll simply add them to this box as well.