aux into my rcd 300 mp3 , cant access aux feature

[jamesey]

hi ive got a rcd 300 mp3 and have bought a usb/aux jack socket thing off fleebay , i cant connect the usb side of it because the plugs are different on the back of my radio but the jack plug for aux clicks into the original plug and fits ok , but when i press cd twice nothing happens , even whan anythings plugged it , it just doesnt work , why??? anyone had this problem