This thread is dead but the problem remains as of June 2010. I have confirmed that this is a DNS issue. Google maintains many redundant servers at differnt IP addresses and depending on your ISP or the position of the stars you could be hitting any one of them.
At the moment Site Advisor on my system at work is functioning correctly but my identical system at home is not. When I ping www.google.com from both systems I receive different IP addresses. If I manually enter the IP address from my work system on my home system I am directed to Google and Site Advisor works correctly. This confirms the DNS issue.
It appears that McAfee's Site Advisor indexing system is hard coding the IP addresses of whichever Google server it reviews and many of the servers are not being indexed. This must be a difficult issue for McAfee to solve since the problem is obvious, has been going on for a long time and they don't seem to want to discuss it.
That's interesting but far too technical for me I'm afraid. The Technical Support Chat staff might be interested in escalating that to a higher level, you could try them. Via Useful Links above.
Note that things have changed lately so try this, it worked for me.
On the Google page go to the "Search Settings" link up top then turn off the "Google Instant" option.
Make sure you have Google saved as a favorite so that the settings cookie does not get deleted when you exit IE.