You should have received a link to the
Meteohub page on our forum when you received your Sheevaplug. The Meteohub credentials are different to the normal Sheevaplug.
IIRC there is a shutdown button on the web interface. That's the best way to achieve a clean shutdown. Wait (say) 30 seconds after you can no longer communicate with the device, then de-power it. In general, the Meteohub is configured in such a way that whenever power is interrupted, it will try to reboot from the SD card as soon as power is restored. It never gives up on trying to boot from SD card, so long as power is supplied, repeating the boot process in and endless loop, until it succeeds.