The handlebar mount doesn't work on a mast, unless you have an adjustable head.
There are two ways:
I like mounting it on the baseplate arm (picture 2) as it seems to give better pictures and also makes it easier to adjust when you are on the water. I also use the velcro strap around the housing normally. You currently need the Gopro tripod mount, but if they release the new adaptor you will not.