How do you get custom WADS working in Mac Doom Legacy?

I'm a Mac user who has Legacy for the Mac. I've tried downloading custom WADS and putting them in the same folder as Legacy. I've even dragged the WADS onto Legacy to try and get them to work, but it doesn't happen. Does anyone know what I can do? I'd appreciate any help.

As far as I can tell (since I don't have a Mac, but I've heard this discussed before) you have to launch Legacy from a terminal window, with the command line switch "-file" - like so:

<program> -file wadname.wad

I could be incredibly wrong about this though, so if it doesn't work, please don't hurt me.

So that means I would enter in <DooMLegacy> -file osp.wad, right? Since the wad I have is called osp.