Per Maryland Form 502 Instructions, page 3:

Baltimore City Residents

Leave the Maryland County box blank. Write "Baltimore City" in the MARYLAND POLITICAL SUBDIVISION box.

In order to correctly calculate the Baltimore City Tax Rate, the County field in the address section must be left blank. In order to do this, please select No County in the County field. This will calculate a blank entry in the Maryland County box on Form 502. If None is entered in the Maryland County field this will calculate a tax rate of zero for Baltimore City.

To enter the information in TaxAct:

1. From within your TaxAct return (Online or Desktop), click State.
2. Click Maryland in the expanded State menu 
3. Click Basic Information to expand the menu and then click County and City
4. Click Continue on the Maryland - Physical Address screen
5. On the next screen, Maryland-County, select No County from the drop down list and click Continue