Okay, let's try something else then. I can't post screenshots, cause I don't have Photoshop on the laptop I have at my disposal now.
1. In the layer palette, click on the Shape 1 layer, it should turn blue, or a different colour depending on your settings. Either way, it should be selected.
2. Now, on the bottom of your layer palette, there are a few icons, 7 in total. The most right one being the wastebin icon, the most left one being the chain icon. Click on the third from the left (the rectangle with a little circle in it). This is the mask icon.
3. You will see that your Shape 1 layer, has this particular icon next to it. You added a mask to the Shape 1.
4. HOLD SHIFT, and click on the PAINT HERE layer in the layer palette. You have now selected everything within the plane. On the Canvas this shows as a walking ant line.
5. Press Shift and hold, press CTRL and hold, and press I. You have now Inverted your selection, meaning your selection is now OUTSIDE the plane.
6. Remember that mask that we made in 2.? Click on it, again in the layer palette.
7. Press B (for Brush)
8. Press D (for Default Colours)
9. Press X multiple times, and see what happens, and where it happens. You will see the fore- and background colour change. The black colour should be on top of the white colour.
10. Press 0 (for 100% opacity)
11. Press Shift-0 just to be sure (won't explain why)
12. Press ]. You will see your brush size get bigger and bigger. When it is a suitable size (I don't have the PSD, so I don't know the dimensions), START brushing on the canvas. Don't worry about brushing inside the plane, in Step 5, we made sure only the outside of the plane is selected.
With the black brush, you are now MASKING everything outside of the plane, keep brushing until you are content.