The Add New Items dialogue allows you insert plugins into your panel.
The Add New Items dialogue can be opened by right clicking on empty space on a panel and going to Add New Items. If you right click on a space occupied by a panel plugin, you can access Add New Items by going to Panel ▸ Add New Items.
To add a panel plugin to a panel, select a plugin in the list and click Add. The panel plugin you have selected will appear in the panel.