[Plugin: NextGEN Gallery] gallery only appears if I create a new page

Hi
I have just installed this plugin, and it is really great.
However, it only shows up if I create a new page for it. For example, i have created a gallery called “test”. I have linked it to a page called vintage bears, where I have placed the code. When I go to that page on my site I see “Gallery not found”. However, if I then create a new page for this Gallery (a child of vintage bears) and place the code in that page, I am able to see the gallery.
Can someone please explain why this is happening? And how can i get the gallery to show on the chosen page?

In addition, can someone please tell me where to change the size of the thumbnails please? Is it in the code for this plugin?

Many thanks.