I don't understand why you receive the reply you did from the collection unit so I suggest you file directly with the court in MD or SC will not help you.
I'm sure you're at the end of your rope so let me share with you this website which contain the forms the state can use to force the support ( http://www.acf.hhs.gov/programs/cse/forms/ )
Federal law requires that states help in establishing and enforcing support when one party resides and/or works in another state. The Uniform Interstate Family Support Act (UIFSA) is the law that establishes cooperation between states. All states have an office, the Central Registry, to receive incoming Interstate child support cases. They review the out of state request to make sure that the information given is complete. The request is then sent to the local child support agency in their state. Standardized forms are used to communicate necessary information needed to process the Interstate case action. Once the local child support office receives the case the requested action is processed.
Under URESA, if a parent had an existing support order, she would have two options. She could have the order registered in the state where the support debtor resided and it would then be enforced. Or she could, as mentioned above, send an interstate petition to the responding state, in which case a new order would be issued consistent with the support guidelines of the responding state.
Since you are in SC, I suggest you go to the court and ask to speak to the clerk of the court. Inform the clerk as to your steps and tell him/her that you need to file a Petition for Support Enforcement under UIFSA - "the Uniform Interstate Family Support Act ". It's federal law that all states must has such a process. Each state will label the form differently and assign a different number to the form but it's a federal mandate.
Another option is that you can attempt to seek free representation based upon the county where the father lives if you go to http://www.courts.state.md.us/family/selfhelp.html
but it may be limited to only residents of that county. If that is the case you would have to retain an attorney or represent yourself.