Go to the Private IPs tab in your Account Settings.
Click on "Purchase Private IPs". You will have the option to select IPs from specific geographic locations and the number of IPs you wish to purchase.
You can purchase up to 5 private IPs at a time.
Once you confirm the purchase, you will be given instructions on how to setup up option custom rDNS for your IP(s).
As soon as the IPs are purchased, they are automatically configured to the account and all email will start to use the IP(s) immediately.