Exactly what version of vBulletin are you using?
I gave that conditional as an example, I seriously doubt it will be helpful to you exactly as is. I see you also posted in the conditional article so it's good you found that, but basically you need to alter the conditional to your needs.
But from what you posted there should be no error so it's either something in your ad code or some mod causing a problem elsewhere.
The conditional without vbAdvanced would be:
Code:
<if condition="THIS_SCRIPT != 'register' AND $GLOBALS[forumid] != 6">
my google ad code here
</if>
But that wouldn't be causing the error you posted.