gksu-nautilus I managed to look at files on my Mac partition via as simple right-click. However, at first journaling was still enabled on the Mac partition. Therefore any copied file would not be copied to where I wanted, but only to the /root directory.
I've noticed that my /-partition is full now. Therefore I went into /root and deleted the copied files (moved them to trash, no terminal). But gparted shows me that the /-partition is still full?!!
(In Ubuntu I only have the /-partition, /home-partition and swap.)