You cannot directly drag onto the ROOTUIELOEMENTCONTAINER you will have to add a GROUP or Transparent container and will allow to drag and drop on to it.
And regarding the Attribute when you created the NODE you might have assigned the dictionary structure on the node which wouldn't allow you to add your own attributes , You will have to use create using wizard --> Attributes from the component structure for the purpose.
Hope this helps you.
Thanks
Phani