Mac has disk utility built in, you can use the erase option to wipe usb then reformat as fat 32 which you may get away with. Failing that I seem to remember from years ago disk utility had the option to delete partition as well. If it still does have the ability to delete and create partitions I would go that route then format ( just make sure you are pointing at your usb stick not your macs hdd or there will be tears before bedtime). Ideally you should use a small usb stick under 8gb is best I use 2gb or 4gb which are cheap as chips, Can't be of any more help with macs as I use linux on all my pcs and gparted is my disk utility of choice. Incidentally there is a version of gparted which can be used from a usb or cd called gparted live but be careful with it if you try it as it will do whatever you tell it to do, you have to be 100% sure you are targeting the usb stick you want to format and not your mac hdd