Box 15 on Form W-2 Wage and Tax Statement must list the state for which the employer withheld the taxes and, more importantly, to which the employer remitted the taxes. If it doesn't, the state claimed will reject the withholdings, as they never received them from the employer.
You must always contact the employer for this information if it is not on the Form W-2 (and yet there is an amount in Box 17). If there is not an amount in Box 17, the state listed in Box 15 would be the state the employer reported the wages to in its reporting.
If you need help reporting your Form W-2 in the TaxAct program, go to our Form W-2 - Entering in Program FAQ.