your search box looks to use a custom javascript, but i dont see a way for it to submit.
you'll need to modify the script used to get it to submit onchange.
I'm not familiar with the script being used in this case, so really cant give much more than that.
it doesn't looks like the action page has been set up either. the action of the search for is to go to:
http://www.usautoparts.com.au/search/
but that page gives a 404 error.
doesn't look like the search functionality was ever completed.