You shouldn't be using GTK+ 1.2 anymore. It's old and deprecated. Some distros even consider removing it completely. For newly-written software use GTK+ 2.0 instead.
GTK+ 1.2 really has no concept of a cursor in a CTree so you have to keep track of the last selected row manually and use that when you receive the "clicked" signal. With GTK+ 2.0 you can simply check the current cursor position in a GtkTreeView and also connected to the "cursor-changed" signal.

You can then fetch the node data and modify the current context popup menu or select the appropriate one from a set.
The nodes themselves don't receive any signal. You could manually delegate the signal to an object stored in the node of course, if you prefer it that way.