all you do is symlink them
ln -s /tmp/mysql.sock /var/lib/mysql/mysql.sock
freebsd's mysql will use /tmp/mysql.sock but plesk wants
/var/lib/mysql/mysql.sock
# locate mysql.sock
/var/lib/mysql/mysql.sock
# ls -al /var/lib/mysql/mysql.sock
lrwxr-xr-x 1 root mysql 15 Dec 9 09:26 /var/lib/mysql/mysql.sock -> /tmp/mysql.sock