when you log in at glitch.com you'll be taken to your home page where a pic of your character is on the left with a button that says 'enter the world' -that is how you get into the game.
when you're playing in the game and want to exit you will see a drop down box at the top right of the screen, choose "exit the world"
You're not exiting "a" world with the option of entering another - where you are in the game world is just where you are. Can you just go back down the tree?
Are you doing "The High Jumper" quest??? I can't remember if that quest has a timer or not, if so you just let the timer run out...if not I'm not quite sure there is a way to get back to the game w/out getting to the top of that tree first.
Try clicking the magic rock. It might offer you a teleport option if you've set a destination. The only cost for this method of teleporting is energy.
Try clicking the map and a location on it, it might offer you a teleport option. This will cost you a teleport token and these have to be purchased with real-world money. (I think this method might not need a token if you've reached a high enough teleport skill level.)
However, both teleporting options might be disabled in special locations.
Usually, if you are sent to a location by the magic rock, and you get stuck or bored, just wait long enough and there will be a small pop-up at the bottom right of the screen asking whether you want to exit the place. It is all right to exit without finishing the quest usually - you can always get back in later via your quest page.