[xubuntu] Keyboard-mouse don't work in grub

I am using an iMac (2006) 24" model. I have three ext4 partitions (sda3 sda6 sda7) and one hfs+ partition for my MacOS (snow leopard-sda2). When I start up my Mac, I press the linux icon in the Refit menu. The Grub menu appear with the linux distros I have installed on my three partitions. I try to select one of the distros to boot from but my mouse and keyboard don't work at all. How can I fix this problem?

My keyboard is a wired usb keyboard.