http://oaktreeinn.com/Home

Looks like you are correct. The Brunswick location does not yet appear in the dropdown menu of locations but it shows up on Google Maps. According to Google Maps, there is nothing else in Brunswick. You have to go to Harper's Ferry.

OP: What exactly about accessing the hostel is puzzling you? There are biking directions from the C&O right on the hostel's web site. Even simply looking at Google Maps satelite view you can see the foot path (part of ther Appalachian Trail) leading from the C&O across the railroad and onto Keep Tryst Rd., which leads to Sandy Hook Rd. which is where the hostel is.