Starting at the main dashboard (“Manage Compute” –> “Instances”), you should see something like this:
When you click on the “Associate Public IP” link you will see something like this (left below), click on the plus (+) sign:
On the screen that pops up (right above) Click on the “Associate IP” button then when you are back at the first screen you should see your IP address in the field, then click “Associate”. When done you should see this:
Notice your new Public IP address is now listed under the “IP Address” Column (highlighted above).