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It isn't possible to load instruments right now, you'll have to set them up by yourself. I'll change that in some way in future versions.
For now, just get the example tunes and see how instruments are made.
The NES sounds almost like a gameboy, with a few (but not unimportant) exceptions.
If you want to create GB music instead, check LSDJ:
http://www.littlesounddj.com
Yes LSDJ isn't free, well if you want the full version it isn't. Acctually you could use the demo-rom on an emulator and by using savestates you can save your work. But if you like it, and want to use it on a real Game Boy and have the save song function, you really should register it!
Loading and saving instruments in FamiTracker is a function I would love to see in the future by the way...