Where is X11's error log located in OSX Lion?
Fire up Terminal
cat /private/var/log/system.log


This will get up the System Log and let you see what is going on. You should be able to find the results you need in there.

If need be, pass that onto the grep command get it to find the strings you want.

cat /private/var/log/system.log | grep [string]

This will output the strings you requrie to the screen specifically.