A special utility called mdl2dxf can be used to convert 3D models from Flight Simulator (.mdl) to DXF CAD graphics (.dxf), which is what in most cases mdl to dxf conversion represents.
However, several other games also used .mdl files for models, so for example Afanche 3D Viewer for Mac should have the capability to export Quake models (.mdl) in DXF format.