whywont my apps download onto my ipod touch, version 4.2.1, says i need to go to itunes and download from there but it never downloads

i have an ipod touch, the generation right before the camera's and mic's, im trying to download a callage ap and everytime i try to download it an error message pops up saying  have to go o itunes and download it onto my ipod from there, but when i go to itunes it doesnt show up or if it does it dont synce to my ipod touch, says itscompatible with my ipod but it ever gets put on it

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