ipod does not open on imac, when I plug in to USB?

WHEN I PLUG MY IPOD TOUCH (4TH GEN 8GB 5.1.1) INTO MY IMAC (INTEL IMAC OS X 10.6.8) NOTHING HAPPENS. IT DOES'NT EVEN CHARGE. AND WHEN I PLUG INTO MY KEYBOARD A MESSAGE POPS UP SAYING DEVICE NEEDS MORE POWER TO RUN PLUG INTO USB PORT ON COMPUTER. AND ANY OTHER COMPUTER I PLUG MY IPOD INTO ITUNES POPS UP AND IT CHARGES LIKE NORMAL. AM I MISSING DRIVERS OR WHAT? AND IF SO HOW DO I FIX? PLEASE HELP.