Complete newbie.
How do I load an image and then apply trace.
If I import an image I get a box with an X through it.

Trance node can import image by itself.
Just create a trace node and then in Trace tab you can define your path of image file by Image Input.
By default it is set to circle.pic
also underneath Trace you can choose COP instead of File, in this case you will need to define a COP node.
With Filters tab, you can manipulate details of input image.