No, don't check the box. Request a corrected W-2 from your employer. If you have a 401k through your employer but the box is not checked, this may have certain tax implications.
For example:
This may allow or disallow you to contribute money into other retirement accounts.
I thought of that too. I did contribute to it and there is an amt in box 12. So my assumption is that I should have a W-2 with box 13 checked if that's the case?