how do you load the US Department of Defense Certificate into an iPhone?
Firstly you'll need to obtain the SSL certificate, sending it to you via email or grabbing it from a web. Once opened (tapping on it from the email attachment, for instance) an installation process will start. 


On the Install Profile screen, tap on Install.


A warning message saying that Installing this profile will change settings on your iPhone will appear. Tap Install now.

 
If your device is Pin Protected you'll be prompted to enter it.
You'll, then, be prompted to enter the certificate's password. Tap on the Next button on the top right corner of the screen, once the password has been entered.


The certificate will be installed, and a screen with the new certificate will be show.