This assumes you have checked out our Getting Started article and you have a dynamic cluster up and running. The mode your dyanamic cluster is using will define how you will go about adding application servers to the dynamic cluster.
You will select your dynamic cluster and then select Dynamc cluster members. Then select New. This will walk you through the process of creating a new applcation server that will be a member of the dynamic cluster. The important takeaway here is that you cannot add an existing application server to the dyanmic cluster in manual mode. You can only create new.