Black window, focussing problem with teleconverter lens for Nikon D3100?

I recently bought a Nikon D3100 and a 2X teleconverter lens along with it.
The standard kit provided with the camera was a 18-55 mm VR.
When I bought it, I didn't really know what a teleconverter does except for increasing the zoom capabilities.

Now that I use it, I'm facing two difficulties, and I'm not sure whether these are problems with the lens or whether this is normal.

1) When it's on 18mm (zoomed out), the photographs taken appear through a circular black window.

2) As I approach 55 mm such that the window disappears, any photographs I take are focussed only on the subject in the centre while the surrounding areas are blurred.

Kindly tell me whether this is normal or should I get the problem clarified with the dealer.

The 2x teleconverter does not fit the kit lens.

It is suppose to fit the high end lenses such as
70-200mm f/2.8 VR2
300mm f/2.8
etc.