You need to insert your Mac OS X Snow Leopard Install DVD and run it up to Options or Customize and then select Rosetta, and it will install Rosetta for you. IMHO: also install the optional QuickTime 7.
Rosetta was automatically installed with Tiger and Leopard, but became an optional install in Snow Leopard. It has been deleted from Lion and Mt. Lion, due to the expiration of Apple's license to use the undertying technology.