In Talend Studio Repository tree view, click on the Joblet Designs node to expand the Joblets tree.
The same way as for the Job Designs, you can create folders via the right-click menu to gather together families of Joblets. Right-click on the Joblet Designs node, and choose Create folder. Give a name to this folder and click OK.
If you have already created Joblets that you want to move in this new folder, simply drag and drop them into the folder.