Terragen doesn't work right at all.
Trying it in a single-player server and it generates a 5x5 area and starts trying to build within that area, no further. Giving lots of brick errors. And lag.
Try either raising the server's maximum bricks per second or lowering the speed at which terragen builds. You can lower the speed in tterragen options. It is called "Tick speed" or something.
Also terragen seems to not like macs. Macs can go cry in a corner for now, but I may update terragen to include mac support.
terragen isn't even nexus's
Maybe you are thinking of LSG