How do you get on to other areas on monster hunter tri 3

simply you follow the map or if you mean by the hidden area that you can pop up on when your online you have to have explorer as a skill (from eating), this would get you shown in hidden area where there is rare ore and bugs, you have to be high rank for that.
but also when killing Deviljho in the Tundra's he would open up (by bashing into it but this is only when he is weak) a hidden area where it sleep when its about to die in area 1 this is where you can fine dragonbone relics