Floating IPs in the dashboard are managed on the Instances page found within the Project| Compute pane. Click the menu under the Actions column next to the instance you wish to assign a floating IP to:

To assign a floating IP, click Associate Floating IP. A wizard will be revealed that allows you to manage floating IP allocations:

If there are no floating IP addresses available for allocation, click the plus (+) sign to create one. Click the Associate button to associate the floating IP with the port.