Is what you are unable to erase drawn in a different color than what you can actually erase on the same frame? If so, on the eraser tool bar settings, you might have "Selective" checked. If you have this checked, and there are multiple colors on your frame, you can only erase colors based on whatever you currently have selected in your color pallet.
As for selecting a frame in the Level Pane...whenever you do that, you are switching away from camera view and the drawing area will typically turn grey (unless you changed that setting) as you become focused only on frames defined in that level. If you had multiple levels, you would also notice that they disappear, but since you only have 1 level, you're only seeing the background change.
Click on the Xsheet Pane (the one with the frame #'s listed vertically) and it will take you back to your camera view, what you are used to seeing.
Hope this helps.