I tried to recreate this and really couldn't. Below is the screen that is displayed for Step 3 on the Approval Step creation.
This is in a Winter '13 Org. The following screenshot is my test setup I used to generate that screenshot:
I researched a bit and couldn't find any information related to removing that option. Can you take a quick look at my approval process setup and tell me if it is similar to yours? Anything glaringly different? If not, I would suggest opening a case with Salesforce support. To do this, click on "Help" after loging into your org. Click "Contact Support". Click "Open a Case". Good luck!