OR, if you hate going through the menus everytime, you can do like I did and enable the USB mass storage by default (instead of that MTP which doesn't work on Mac OS).
- put this in your init script (if you have a kernel which supports init scripts, currently only CF-Root works on ICS).
setprop persist.sys.usb.config "mass_storage,adb"
- add/change the same settings in build.prop (you probably have now "persist.sys.usb.config=mtp,adb".
And reboot. When you connect your cable, it will default to USB mass storage mode just like it did on Gingerbread.