If you are setting up a CLC Server with CLC Job Nodes, you need to set the "Server mode" at the top of the Job Distribution section to MASTER_NODE, rather than SINGLE_SERVER or EXECUTION_NODE. Please note that you only need to attach job nodes through the Genomics Server web admin interface on the master node.
 

The Genomics Server on the master node also requires a valid server license in order for the "Attach Node" button to work. Without a valid server license, any Genomics Server software will be automatically put into the execution / job node mode and it will not be possible to add other job nodes.
 

If the Attach Nodes button remains greyed out, as shown in the image below, please click on the button labeled Save Configuration in the Server mode section. 
 

After actively saving, the Attach Node button should be available again.