First download and install X11 from here (select the dmg for Snow Leopard):
Then download and install Inkscape from here:
Inkscape. Draw Freely.
On my Mac Lion, X11 automatically launches when I run Inkscape. It's sort of a plug in that's required for the user interface (aka GUI that enables the use of a mouse and keyboard within the application).
It's a lot less complicated than it sounds. I hope it works well for you. I don't how I would survive without Inkscape!