The 3D View is not the only place you can add keyframes. The Graph Editor actually gives you much better control over adding and manipulating keyframes:
In the 3D View, select the camera.
Using the arrow keys, move the current frame indicator to frame 1.
With the cursor over the Graph Editor, use Ctrl+MMB to zoom the Graph Editor out so that you can see at least -10 to 10 on the vertical scale. Look at the left side of the Graph Editor. It's blank.
With the cursor over the 3D View, press the I key and make a Locations keyframe.
In the Graph Editor there are three colored lines, red, blue, and green, just like the 3D axis indicator. Since you have just made a Locations keyframe, these are F-Curves for locations in X, Y, and Z. Use Ctrl+MMB to zoom in until you can see the red and blue lines clearly.
Use the arrow keys to move the current frame indicator to frame 20. Note that you can see orange dots at frame 1 on these lines. They are the...