Create Certificate Signing Request
Start by creating a .certSigningRequest (CSR) file on your Mac, using Keychain Access. Open Finder, and then open Keychain Access from the Utilities folder.
Next, open Keychain Access > Certificate Assistant > Request a Certificate From a Certificate Authority.
Enter the email address that you use in your Apple developer account, and enter a common name. The common name can be anything you want, for example a helpful descriptive name like "ios-mybiz". Check Saved to disk and Let me specify key pair information, then click Continue.
Give your CSR a helpful descriptive name, such as iosapp.certSigningRequest, and choose the location to save it on your hard drive, then click Save.
In the next window, set the Key Size value to 2048 bits and Algorithm to RSA, and click Continue. This will create and save your certSigningRequest file (CSR) to your hard drive.
In the next screen your certificate creation is verified. Click a button to view it, or click Done to go to the next step.
You also get a corresponding public and private key pair, which you can see in the Login > Keys section of Keychain.
Double-click on your new private key to open the Access Control dialog. Check Allow all applications to access this item.
Now login to the Member Center on https://developer.apple.com/. Click Certificates, Identifiers & Profiles.
Then click iOS Apps > Certificates.
Next, click the add button (the little plus sign) in the top right corner of the iOS Certificate page.
Under "What type of certificate do you need?" check App Store and Ad Hoc, then click the Continue button at the bottom of the page.
The next screen, About Creating a Certificate Signing Request (CSR) has information about creating a CSR in Keychain Access. You already did this, so go to the next screen. "Add iOS Certificate", to upload the CSR you already created, then click the Generate button.
Your new certificate is named ios_distribution.cer. Download it to your Mac; then find it and double-click on it to install it properly in Keychain.
After installing it, you should see it stored with its corresponding private key in Keychain.
Remember to make backups of your keys and certificates and keep them in a safe place.