Where is clamscan? [default is /usr/bin] /usr/local/bin
Setting ClamAV location in /opt/MailScanner/etc/virus.scanners.conf
You may need to add /usr/local/lib to the directories searched
for run-time libraries. Read the man pages for ld.so and/or ldd.
The symptom will be that the testing stage of the Mail::ClamAV
Perl module fails completely.

Setting a soft-link from spam.assassin.prefs.conf into the SpamAssassin
site rules directory.
spam.assassin.prefs.conf is read directly by the SpamAssassin startup
code, so make sure you have a link from the site_rules directory to
this file in your MailScanner/etc directory.
Perl could not find your SpamAssassin installation.
Strange, I just installed it.
You should fix this!