On the right side of the main Cedit window, you can see the list of all concepts that were used anywhere in the file.
These concepts are of two types:

1. Concepts with gray color: these are concepts used in the current sentence. If you click them, the corresponding node in the graph area will be selected.
2. Concepts with black color: these are concepts not used in the current sentence. If you click them, nothing happens, but if you double click one of them, the node will be inserted to the top left corner and you can use it in the sentence.

Notes:

1. You can distinguish reused nodes by their border: it's dashed for the concepts which aren't connected to the sentence tree, i.e. also for the reused nodes.
2. On my computer, the double click has to be really fast. If you have problems with this too, let me know.

CAUTION If you change your mind and decide that you don't want the node in the sentence, don't delete it. This would also delete the same concept in previous sentences. Instead, drag it to the concept list on the right side. This will hide it without deleting.