How do I fix a libX11.6.dylib version error

Hi, I'm running Snow Leopard (10.6.8) on my MBP.

I'm trying to install Siemens NX, an engineering CAD system.

I am asked in the install instructions to install XQuartz and OpenMotif, which I have done.

After installing NX I try to start it, it flashed in the bar for a minute or so then disappears.

After searching for the error, I cam across the following message:

/Applications/Siemens/NX9.0/ugii/ugraf ; exit;

dyld: Library not loaded: /usr/X11/lib/libX11.6.dylib

Referenced from: /Applications/Siemens/NX9.0/ugii/ugraf

Reason: Incompatible library version: ugraf requires version 10.0.0 or later, but libX11.6.dylib provides version 9.0.0

Trace/BPT trap

Can anyone help me resolve this error please ?

Thanks

Gavin