Where can my "Disk utility.app" be gone ?

Hi everybody,

I finally post my question, cause I'm so so lost.

So the thing is :

- I bought an Iphone 5 (great!!!)

- but I'm still on Leopard 10.5.8, so I need to upgrade to be able tu upgrade I tunes... etc...

- I bought (yes, I bought!) the MACOS Snow Leopard install CD

- My internal disk driver is dead and I have so far no external disk driver... so it can't read the install CD

- I have my Mac for 5 years now and I have never set up "Time Machine"

- I bought a new Western Digital external Hard Drive... so now I am ready to save all my stuff and to upgrade

1. Desktop recognize the new external harddrive

2. Whenever I try to configure Time Machine / Time Capsule it does not recognize any disk. it launches the Airport utilities... but then tell me it's not recognised

3. I thought maybe I need to configure my new hard drive, so I went on the Western Digital Support telling me to use my "Disk utility.app"... but it's just not on my computer (i have searched with Spotlight, directly in the application>Utilies folder... nothing

in French, we'd say that I'm turning Goat 😟

What can I do ?

thanks so much for your help

Hélène