As i understand, you have a NXT 2.0 kit, and want to build Tribot (as http://www.ro-botica.com/img/NXT/Build-Tribot.pdf
), which is a NXT 1.0 robot.
The inventory of NXT 2.0 lacks some 1.0 specific pieces, and in your case, the "Bionicle Weapon Pincer Suukorak ", item code 50914, which is your arm.
You can find it in briklink (http://www.bricklink.com/search.asp?itemID=54753
), or into some sets: (http://www.peeron.com/inv/parts/50914
). The NXT 2.0 has some other bionicle parts, as the zamor's launcher, magazines and balls, but not those "arms".
You can use a technic liftarm as spare part , the "Technic Beam 3 x 3.8 x 7 Liftarm Bent 45 Double ", Item 32009 (http://www.peeron.com/inv/parts/32009
), to be able to grab the NXT 1.0 balls, but you will still lack the red and blue dacta/technic 52mm balls, NXT 1.0 specific too...(http://www.peeron.com/inv/parts/41250
Hope this help