Go to https://developer.apple.com -> Member Center -> Certificates, Identifiers & Profiles -> Identifiers.
If you want to edit an existing App ID, select 'Configure', otherwise select 'New App ID'.
After you entered a description for this App ID and the Bundle Identifier (use the same reverse-DNS name as in the Twixl Distribution Platform) you can configure it.
When in the App ID detail window, you'll notice a list of all App Services.
Enable for Apple Push Notifications.
When you activate Push notifications for your App ID, you'll notice that Push notifications can be configured.
From there on, you can either generate a Development SSL certificate (for use with a Development test app on the Twixl Distribution Platform) or a Production SSL Certificate (for use with a production app on the Twixl Distribution Platform that you'll be submitting to the App Store).
Create a cerificate signing request in the Keychan Access application on your Mac.
Download your certificate, then double-click the file with extension .cer to add it to your Keychain.
Open the Twixl Publisher application, and select "Export Push Notification Certificate…" from the Tools menu.
Select the push certificate for your app, then enter a password (you will need it later), and save the file, that will have an extension .p12
On the Twixl Distribution Platform, navigate to your app, then select Push Notifications. In the settings, upload the .p12 and make sure to enter the correct password.
Make sure to upload a Development Certificate in a test app, and a Production Certificate for a production app.