A few things are needed for us to help
what version of ubuntu is this ?
hfs+ should just auto mount if it is a usb drive
If not , then you have somethings turned off or not installed
then with the usb drive plugged in run
--- type in the root password when asked ( or use "sudo" instead of su - ) ---
--- and this ---
those two commands will show what the usb drive is and just how it is seen by the OS