little phatty stage 2 in logic 9 problems

Hi i was wondering if any one could help me, i have just got a little phatty stage 2 and want to connect it with logic pro 9 on my mac book pro so i can record and sequence, i have tried connecting the usb to the mac and the audio through the sound card but it seems to make the lp go crazy and says fatal error, keyboard over run! could some one please tell me what i am doing wrong. i have checked youtube ect for videos and can only find people using it witin logic as an external instrument but not telling how to actually do it. if some one could give me a step by step instruction i would be very greatfull!!
Thanks!!

Lee.