Snow Leopard and Lion with the same computer

Hi yall.

As many, I installed Lion on my iMac.
Great product so far but some problems appear.
For me, mostly the lack of Rosetta which unable the use of programs such as Microsoft Office 2004 for Mac.
I installed a version of Mac OS X Snow Leopard on an external disk and it works fine.
I own Parallels Desktop 6 for Mac (which I previously used to run Windows programs on my old MacBook Pro) and I thought it would be nice to be able to run Snow Leopard as a virtual machine so I could use Microsoft Office 2004 for Mac without reinitialize my iMac to switch operating system.
I tried but Parallels told me it couldn't find my Snow Leopard.

Anyone can help me?

Thanks

Jean