In order to export the iOS Certificate with its private key in Keychain Access, please follow these instructions:
1. In your Mac OS X, go to Applications > Utilities > Keychain Access.
2. Select Certificates on the left panel of the Keychain Access window.
3. Expand and Find the Certificates along with its private key.
Note : For Development Certificate you should select the iPhone Developer Certificate and For Distribution you should select the iPhone Distribution Certificate.
4. Select both of them and right click. you can find an option to export 2 items. Click on it to proceed.
5. Provide the name of your certificate and save it to the desired location.
6. Give password to secure the private key and certificate and click on Ok to save.
7. All Done. Now you can find your certificate at the location where you have save it.