A .p12 file is simply a certificate file and private key bundled into a single file.

NOTE: These instructions require the use of a Mac running OSX

1. Open a browser window, and go to http://developer.apple.com. Click Member Center, log in, then select Certificates, Identifiers & Profiles and click on the Certificates link located in the left hand panel. 


2. Select the appropriate certificate, then click download.  Once downloaded, double click on the file to install it on your Mac 

3.  Open your Keychain Access application (found under Applications / Utilities).

4. Select the Keys category.
5. Find the key associated with your Distribution Certificate and click the triangle next to the key in order to show the certificate. Control-Click on your Distribution Certificate and click Export in the menu. The private key is identified by the iOS Developer: <First Name> <Last Name> public certificate that it is paired with.
6. Save your key in the Personal Information Exchange (.p12) file format.
7. You will be asked to create a password but LEAVE IT BLANK then click OK.