Why can't I see the invoice number?

I'm trying to set up invoices and edit the first invoice number.
The instructions say to
1) click on the invoice tab (OK)
2) click on new invoice (OK)
3) click on the settings box next to the Invoice Number (NOT OK!)

The invoice number is not there. I've checked in the layouts window in Setup and it is there, but it is not present on the new invoice screen.

What do I need to do?

Pete