Easy. You have to be approved by the head of the Avatar program, however. When you get your approval, you may link anytime you want. Keep in mind YOU ARE NOT NA'VI. You may NOT go to Hometree without a warrior's say-so or the Clan Leader themself.
While as a human, you may go out into Pandora, just don't leave without somebody else or you may get hurt or killed.
While as a human, you may go out into Pandora, just don't leave without somebody else or you may get hurt or killed.