are you using the cms?
if so i can tell i can tell you
go to admin cp
go to vbulletin CMS
go to widgets
click on create new widget
click on widget type - scroll down to static html
give it a name
click save
then go back into widgets
click on configure
imput your google search box code
heres mine to test it on yours
<form action="http://www.google.com/cse" id="cse-search-box" target="_blank">
<div>
<input type="hidden" name="cx" value="partner-pub-8038068168054371:cetq3s-a66t" />
<input type="hidden" name="ie" value="ISO-8859-1" />
<input type="text" name="q" size="31" />
<input type="submit" name="sa" value="Search" />
</div>
</form>
<script type="text/javascript" src="http://www.google.com/cse/brand?form=cse-search-box&lang=en"></script
click save then add the widget into your CMS by clicking on layout manager.
If its for getting on your forum i cant help with that, newcomer to VB4 myself.