Per Maryland Form 502 Instructions, page 4:

Baltimore City Residents

Enter the name of your county on the MARYLAND COUNTY line. If you lived in Baltimore City, leave the MARYLAND COUNTY line blank.

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. 

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