Does the Dedicated IP appear in your list of IPs in Tools & Settings > IP Addresses ? If not then you'll need to add it.
You then need to open the Subscription for the domain (i.e. either type the domain in the search box at the top right of the page, or just find it via the Subscriptions link in the main menu on the left).
Next, click on the Change Hosting Settings button you'll see in the top menu for the Subscription (Change Plan | Custoomise | **Change Hosting Settings** | Suspend | Change Subscriber | Remove)
In there you'll see a drop down that will allow you to select from *unallocated* Dedicated IPs.
Note that all domains within the Subscription will use that IP.
In doing all this, in a way, the Dedicated IP has been allocated for the sole use of the Customer that the Subscription belongs to. What this means is that if you add a new Subscription to that Customer, you'll find that you are able to pick the Dedicated IP you previously allocated to the first Subscription (or one of your Shared IPs if you prefer) for use by the new Subscription that you are adding to the Customer.