If its the Modnation one you want that auto updates the MOD image in the SIG of what you last raced with -
Goto the official Modnation.com website
Login with your PSN login
Click on the Community Tab
Then Click on the Signature Tab that appears on the Left of the page
Copy the HTML code it generates.
Login to this Forum and goto your settings, and paste the code into you signature field.
And thats it. Presto!
The signature may not show until you have loged into play Modnation on the PS3.