The Apple Software License Agreement (SLA) only allows virtualization of the "Server" flavors of Mac OS X 10.5 and 10.6. Apple lifted that restriction in 10.7 onwards: They now allow the virtualization of Mac OS "Client" versions from Lion onwards, but they didn't retroactively allow it for the older versions, so for your existing Snow Leopard disc, I'm afraid you're out of luck.
Only a month ago, though, Apple was selling Snow Leopard Server for $20 in the US and Australia by phone order only... see this thread (towards the end of the thread in particular!): Running Snow Leopard in VMware on Mountain Lion Host. I'm not sure if that offer is still going.
Cheers,
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Darius