OnMouseDown is a Unity function. NGUI equivalent is:
void OnPress (bool isPressed)
Check the documentation for UICamera.
http://www.tasharen.com/forum/index.php?topic=6711.0To create in-world GUI, do the following:
1. New Game object (ALT+SHIFT+N)
2. Attach UIPanel to it.
3. Scale it to something small, like 0.01.
4. Add widgets to it like you normally would (ALT+SHIFT+S to add a sprite for example).
5. Position this game object where you need it to be in the world.