setting up samson co1u in logic

Hi, im running on a brand new imac os x 10.5.2. and logic studio, and i cant get my samson co1u to work. when i go to the midi set up folder in utilities, aggergate doesnt come up. the only ones that appear are built in microphone, built in input, and the samson co1u. i have tried many thing and read a lot but nothing is helping. thanks in advance