Yes, checking the "active duty pay" box for your W-2 is the first step.
The second step is when you get to the Maine state return:
- On the "We've Recommended a Maine Residency Status" screen select "Change Residency"
- On "Change Your Maine Residency" select "Safe Harbor Resident"
- Continue through the questions
- Verify that your military pay is entered on the "Nonresident Military Compensation Subtraction" screen