>I keep getting an error that says, "Your system's Java Virtual Machine is incompatible."
The Worlds client needs Java 6 to run.  You can download this at http://users.worldsonline.com/users/jimbly/java/index.htm

>Worlds doesn't start up for me at all or begins to load then shuts down!
Try running the program as an administrator.  If you are using Windows 8, you're basically screwed until someone figures out a fix.  It is very unlikely that there will be an official patch or anything of the sort for Windows 8.

>When I try to enter a world, I am denied access./"...it is a custom world that is not available to you."
Your account doesn't have the proper permissions to enter this area.  This can be because it is a VIP-only area, or sometimes because it is a user-made world that only allows certain users in.  There has been an incident where a person was missing vital program files.

If you have an AMD card: "Disable your display adapter by going to Device Manager then Display Adapter. If you disable it, the screen will become small but you will be able to load Worlds. Make sure to enable it once you log onto Worlds so you can play it in fullscreen."