Read the "teensyduino" section of the teensy site - that has the instructions for getting the IDE extensions you need installed.
I suspect you can't just call it a leonardo because the boot loader is probably not the same as the leonardo.
I think if you really want to use the Arduino IDE, you could flash the Leonardo boot loader on it, if you have a programmer. You should also give WinAvr a shot, its pretty awesome once you get use to it.
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