3D models saved in any of the known MDL formats can be opened and processed in 3ds Max (previously 3D Studio Max) and saved in the default 3DS format used to store 3D graphics. Some MLD formats may require a special plugin, but the common variants should be supported by the program by default.
You can also use some of the 3D model / CAD conversion software and services which should be able to perform mdl to 3ds conversion.