I believe I had the same problem. Either look up the filename that comtes with the error, or look up .NET framework update. And there you go. Either that, or do it the long way.
Make a new folder in the SRB2 main folder called MD2. Then create a file called MD2.dat in the SRB2 main folder, not in the MD2 folder. It needs to be created first in text format. Then place the models and skins inside the MD2 folder and put the code that comes with it in the MD2.dat file. When that is done, go into SRB2 OpenGL and type in the console (usually the ~ button) GR_MD2 ON. Then test it ingame. Try Boinciel's Good Models first, trust me, they ARE good.