mds huh? What does that mean? Lol. Well I am pretty sure you mean MDL. So... get Milkshape, which is a modelling program. Ver-1.6.6. Once installed, place the MDL file somewhere (Folder would be good choice)
Go into Milkshape go to tools -> decompile normal HL blah blah.. Once decompiled go to file->import->(the SMD of the model, name.smd)
After that you can go into photoshop, file->open->(search for folder where you put model) once found you we will see all the (BMP) files in there, open from there and edit... That's about it. Also theres another easy way... Download HLMV (Half-life Model Viewer) which allows you to look at the MDL files.. There you can also edit the skins..
Hope this helps.