Cannot Install Snow Leopard on Macbook Pro 7,1

Hello guys,

I've recently bought a USED Macbook Pro, which has installed Mac O SX Lion (10.7)

The thing is I need to install Snow Leopard and I can't do it. I have tried by simply clickin on the Installer Icon and also by booting pressing the key "Option" but when I do this the system asks me to restart my Macbook.

The DVD I'm using a Original Copy of Snow Leopard 10.6

Any ideas? Please! Help me!

my Macbook Pro:

Macbook Pro 7,1

Intel Core 2 Duo 2.4

4 GB Ram DDR3

HD 250 GB Hitachi HTS545025B9SA02