In general, to add a node, go to the Physics toolbar, no matter what operating system you are using. Subnodes are available by clicking the parent node and selecting it from the Attributes menu.
Volume Forceis described for the Pipe Flow interface in the Pipe Flow User’s Guide.
In the COMSOL Multiphysics Reference Manual see Table 2-3 for links to common sections and Table 2-4 to common feature nodes. You can also search for information: press F1 to open the Help window or Ctrl+F1 to open the Documentation window.