If thats all the commands your using your in bad shape. I suggest you do some reading at the aircrack site and then in the tutorials section there are some very fine videos by my friend xploitz on how to accomplish what you are trying to do. I suggest reading and understanding how it works because when you get in the field any number of things can happen. If you can't follow the video than you may be in the wrong field. If you post ALL your commands maybe we can see where you went wrong.