Rosetta won't work in Snow Leopard?

I have a partition on my Mac Mini running Snow Leopard. In fact I reinstalled it last night from the original disk and made sure that Rosetta was checked under optional installs as I want to run a classic program. I did not do any software updates. I inserted the CD with the installer program (classic) and I get the null symbol over the installer icon as though it were incompatible.

Why, if Rosetta is installed, can't I run a classic program?