Linux does not support mounting a Journaled HFS+ volume read/write, at least not safely.
As Mac OS X by default (for many years now) has formatted HFS+ volumes with journalling enabled, this is a problem.
If it is an external data drive (for Movies etc) that you need to move back and forth between the Pi and a Mac you could format it as HFS+ without journalling and it should mount read/write. (I have not tested this however)
Or you could use EXFAT / FAT32 which are supported for read write for both Mac and Linux.
One final possibility if you can’t reformat the drive, is that you can force remount the drive as readwrite using -o remount,rw, however this is not recommended as it can lead to data loss, and we don’t provide any automated way to do this at the moment, so you would have to do this manually every time the drive was attached, or script something to do it - making the entry rw in fstab is not enough.