To assign an Approver to a Deployer, simply locate the Deployer using the User Search functionality demonstrated in Search for Users.
Select a user from the Approver dropdown box. Any attempt by the Deployer to create a deployment with a value greater than the Threshold that you have set for that for the user will then be referred to that Approver for action, as described in Approvals.
NB. Once you have assigned an approver don’t forget to select Save Changes to ensure that the change takes effect.