Ok. First, open notepad. It's under accessories in the start menu.
Type in
PLAY PLAY.MD2 3.0 0.0
or copy and paste.
Then, you have the files, don't you? PLAY.PNG and PLAY.MD2. (or player.md2. I'm not sure which)
EDIT: They go in the folder "md2". Make it, it's not with SRB2.
Use OpenGL mode, and type in the console GR_MD2 ON.
There you have it, 3D Sonic. If needed, I will revise this, if you don't understand well.