Check out our getting started guide:
http://www.redhoundsoftware.com/apps/smime/index.html
Explanation for required permissions:
* Take pictures and videos: you can use your phone's camera to scan QR codes instead of entering passphrases manually
* Internet Access: If you enable it in the settings (it's off by default) you can send error reports to our support server.
* Storage Access: You can use Secure Email Reader to save attachments to your SD card and read keys from your SD card.