Apple forgot to include a "uninstaller" toolkit in Mac OS X, so to
install the older package you need to manually uninstall the old one.
I think this will uninstall the old install properly. Note that if
you have database files under "/usr/local/mysql/" you need to back
them up
rm -fr /Users/*/Library/PreferencePanes/MySQL.prefPane
rm -fr /Library/PreferencePanes/MySQL.prefPane
rm -fr /Library/StartupItems/MySQLCOM
rm -fr /Library/Receipts/MySQLStartupItem.pkg
rm -fr /Library/Receipts/mysql*.pkg
rm -fr /usr/local/mysql*
After that you should be able to install the DMG.
You can read about Mac OS X for PowerPC and the ended support
http://www.mysql.com/support/eol-notice.html
I have no idea why the already released packages where removed
from the download pages.
You *can* find MySQL 5.1.40 packages for Mac OS X and PowerPC
on the (not that easy to find I have to admit) page
http://downloads.mysql.com/archives.php?p=mysql-5.1&v=5.1.40
Sorry for the inconvenience,
kent
Kent Boortz, Release Staff Engineer
Oracle, the MySQL team, www.mysql.com