Hi Yuriko,
I think the TGPGuiLib should work fine on a 64 bit system. It's only 32 bit but TG Classic is only 32 bit so that's not a problem. There is one thing I can think of which might be different. On the TGPGuiLib website I say that the default install location for Terragen is:
C:\Program Files\Terragen
However on a 64 bit system that will very likely be:
C:\Program Files (x86)\Terragen
That might be your problem. If that doesn't help could you please give more information about what the problem is that you're having?
Regards,
Jo