I wouldn't use the old SDK, 2013 works well. First, follow the directions on the valve wiki for setup/build.
Running the game with yout mod is not well documented on the wiki, but generally all of the tools take a -game argument that should point to the path of your mod directory. For an SDK...
Hey optical:
I think the physgun is already in SDK 2013/Multiplayer:
src/game/shared/hl2mp/weapon_physcannon.cpp
The mega physcannon isn't in MP, but you could probably port the code. I happened to borrow the looks/animations/sprites from the mega physics canon in my own mod, but my...