The short answer is you cannot - it works only when the device is connected over WiFi.
Quoting from iCloud: My Photo Stream FAQ (emphasis at the end mine):
When do photos upload to My Photo Stream?
When you enable My Photo Stream on your devices, all new photos you take or import to those devices will be automatically added to your photo stream.
iOS devices: New photos you take are automatically uploaded to your photo stream when you leave the Camera app and are connected to Wi-Fi. My Photo Stream does not push photos over cellular connections.
Such limitations are usually imposed by Apple because cellular Internet is quite expensive in most places when compared to DSL/cable broadband. So uploading photos that are a few MBs each could quickly run the bill up to a hefty amount. Another example of limitations on cellular connection is App Store downloads, where iOS will only download an app or update if it's smaller than 100MB in size when the device is connected over cellular.