What to do if APN settings appear disabled
Sometimes, carrier-provided settings that are loaded onto a device may be "locked" such that they can't be modified. While this is often an indication that you shouldn't need to modify them, it is sometimes possible to work around this.
At the APNs screen, try the "Reset to default" option available by pressing the menu button. This will delete any APNs you have customised yourself, but sometimes this can also be used to remove locked APN settings that have been loaded from your carrier.
You may also try removing the SIM card and/or entering "Flight mode" prior to selecting this "Reset to default" option. Note that your Android device may require a reboot after removing the SIM card.