I live in DC and worked in MD. My employer was not withholding state income tax from DC or MD. Do I only need to pay DC state taxes on that income? How do I do that?

MD and DC have tax reciprocity with regard to W-2 wages.  If you are a full-year resident of DC and you work in MD, your W-2 wages are not subject to MD income tax.  You need only file a DC tax return.  For tax purposes, your MD earnings are considered DC income, and are taxable by DC.  If you entered MD in the Other State Income section of Personal Info, change it to No Entry.