This article relates primarily to users running Plesk 10 on our Cloud VM plans.
First you will have had to have purchased\set up a new IP address on your VM itself:
Once you have done that note down your new IP address and log in to your Plesk control panel. Then go to:
Tools & Utilities > IP Addresses > Reread IP
Plesk should then automatically pick up any new IP addresses you set up on the VM. It will most likely set it to dedicated by default so if you want to change that just click on the new IP address itself and change it to Shared if you want.