What prevents the iMessage slider from working on a new iPhone 5?
You will need to work through the various causes at will contribute to the failure of your device to register for iMessage service. 

My guess is there is something wrong since normally, you get signed in and find a password error when entering your Apple ID elsewhere on the phone. Also, a problem with SMS activation happens after you manage to get the phone to accept the slider in the ON position.

Start here http://support.apple.com/kb/TS4268 in troubleshooting and know you can also call Apple (or visit a convenient store) for more personal help. New devices, no matter where you purchased it, get free support for setting it up in the first 90 days.