During upgrade to Parallels Desktop 7 all Mac printers are installed to your Virtual Machine (read more about Virtual Printing
feature). But after reverting to the snapshot made in Parallels Desktop 6 all printers are gone.
Printers are added to the virtual machine during Parallels Tools installation. The snapshotted virtual machine still has old Parallels Tools installed, and this configuration has not been made yet at this stage.
Start Parallels Tools update from Virtual Machine menu
→ Update Parallels Tools
and it will install all printers again.