Step 1: Open up Flash and start with a blank document. I used the default settings, but you can use whatever you like. Grab the circle tool, turn off the outline and leave the fill color black. Now create a circle on the stage.

Step 2: Select the circle that you just created and convert it to a graphic symbol (Right-Click > Convert to Symbol). I named this new symbol 'ball', but it you can call it something else if you really want.

Step 3: Go on over to the toolbox, grab the pencil tool, and set the tool's drawing color to black. Then go up to the layer panel, and click the Add motion guide button.  The location for this button is clearly illustrated in the image on the left.

Step 4: You should now be presented with a new guide layer. Select it and draw a path around on the stage.  This will directly correspond to the projected path on which your object will travel.

Step 5: Choose the Selection Tool, and then go to the bottom of the toolbar to make sure that 'Snap to Objects' is on. If it isn't click it and set it on (it looks like a little red magnet).

Now select the circle symbol, and look for a small ring in its center. Click and hold on it, and you can drag the circle. Try putting the ring over top of the path. It *should* snap to it automatically.

To finish this step, snap the circle to the line, and place it on the line in the location you would like it to start at the beginning of your animation.