When viewing your list of customers click on the "View Inactive" button, this will produce a list of your inactive customers. Find the customer you wish to re-activate. The last icon under the "Actions" column is an "". Click on it to re-activate the customer. Click on the "View Active" button and the customer you just re-activated will be at the top of your list.
You can also search for a customer by entering the name, email or phone in to the "Search Customer" field.