Can someone help me? how to unlock forest?
Well FIRST from the Lab is going to the very rightmost of the abandoned town, look for an entrance of any sort near a body of water, enter to meet a Small Scruffy Bandit, beat her as you like or Don't even try actually, give her assurance as an ally... Or FISH!!!
After that she will show you the location of her ol' safehouse, follow directions (It's near the exit) to find her safehouse, enter go upstairs and loot the chest for the key to enter the forest.
This is also the path to find Transylvania so prance around aimlessly in the forest for a screaming plea, or cries of ticklish pleasure/pain.
Further on...
Spoiler
.
.
.
.
.
.
.
.
.
Facing the Gaping plant the wall right of you has a hidden entrance.
Just dive face first into the wall till you find it, you need to BEAT not lewd the fairies, so a good old trashin will do the trick and as they flee in fear realizing they won't be getting any loving kisses you can collect their Powder, now enter the hidden grove and pour the Powder along the statue, as you enter they will desire a gift, they Love milk, proceed to play with the Queen, after a little playtime you can ask her about the gaping plant, this needs a fully refurbished Milk Farm, as you need cheese to pass her test and that needs the Refinery.