Flash MX 2004 - Motion Guides
Adobe Flash CS3
Adobe Creative Suite 3
Start off by opening a new document, default size is fine.
We're going to first create the symbol that will be moving along the path. Go to Insert > New Symbol (or Ctrl + F8). Name it circle and choose Graphic and hit OK. To keep it simple we're going to make exactly what we named the symbol, a circle. Select the Oval Tool (O) from the tools palette, hold down Shift on your keyboard and draw out a small circle about 50 pixels wide. You can gauge this with the Info Panel (Ctrl+I), paying attention to the W/H (width and height) values. Or you can simple enter 50 into both of the input boxes after you've drawn out some kind of circle.
Choose the Selection Tool (Y) and left-click the circle and align it centered. You can do this with the Align Panel (Ctrl + K) and choosing the Align horizontal center and Align vertical center buttons.
Go back to Scene 1 (button above the Timeline) and from your Library, drag the graphic on to the stage. Click it and drag it off to the side because we don't need it right this minute. Left-click on Frame 20 and insert a keyframe (F6) and do the same at Frame 40. Go back and left-click Frame 1 to select it and in the Properties Inspector (Window > Properties or Ctrl + F3 if you don't see it) choose Motion from the Tween drop-down menu. Do the same at Frame 20 as well.
Now we're going to create the motion guide that the circle will follow along.
Click the Motion Guide button at the bottom of the timeline (
). This should add a Guide layer above your original layer. Select the Pencil Tool (Y) from the Tools palette and draw out a simple wavy line, something generally similar to this:

To make sure you don't accidentally select this layer you can lock it by clicking the layer right below the small padlock on the timeline.
Now with the Selection Tool, left-click the circle making sure only it is selected if you didn't lock the guide layer. Click on the registration point (the small circle with the crosshair) in the middle of the circle and drag it to the left end of your motion guide. You should see the circle snap to the end of the guide. Left-click on Frame 20 and this time click and drag the circle to the opposite end of the guide, making sure it snaps to the end of the guide. Finally, go to Frame 40 and drag your circle back to the left end of the guide making sure it snaps in place.
That's it, test your movie (Ctrl + Enter). If all went well your circle should travel to the right along the guide and then back to the left where it started from.