You probably have to make one. It goes in your home directory. It's easy.
1. Open your terminal
2. Make sure you are in your home directory by typing cd ~/
3. Type "pico .xinitrc" (without the quotes)
4. Type in the following lines:
PATH="$PATH:/sw/bin"
xterm -font 9x15 &
Eterm --font 9x15 --scrollbar=0 &
/usr/X11R6/bin/quartz-wm
5. Type Control-o, hit return, type control-x
6. Close your terminal and launch X11 - it should work.