well... for this u must do something at the first visit to the labyrinth...
at the Labyrinth B room, when u enter it for the fisrt time, u see Professor Booster falling down. u must ignore him and just cross the gap to the next rom jumping (there's a red mark on the floor to indicate the moment of jumping - u must be runing) so save on the disket of this rom and keep trying to cross to the next rom (where u'll face ballrog). so when u leave the labyrinth and return to the mimiga village, professor booster will appear at arthur's house from the teleporter, and u'll be able to return to the labyrinth. Professor booster wii also give u the Booster v2.0 (this one let's u boost to any direction), with it u can go for the best endig of the game.

You can get back to the Labyrinth through the main teleporter in Arthur's house if you ignored Booster when you saw him in the Labyrinth. Or you can get back through a teleporter in the Plantation, the one at the bottom left will take you to a point within the Labyrinth but the first time you try to use it you will be knocked out and have to do some story related stuff first.