First off, you have to make your mask. Your mask can be anything (a shape, text, etc). For this example I will be using the letter T. Put the mask on top of the object layer.
Right click the mask layer. Select MASK from the drop down menu.
Notice that the icons changed from the page icon to a blue-green sort of look. They also locked themselves. This is important. In order for the mask to appear that it is masking anything at all it has to be locked along with the object layer. Unlock the mask layer now.
Create a motion tween on the mask layer by clicking the layer and going to the toolbar, clicking INSERT and dropping down to MOTION TWEEN.
Animate as desired.
Relock the mask layer.
Play the animation by hitting ENTER or using the shortcut COMMAND+ENTER to export to a SWF and thats it!
Go to this link to download the original file:
Enter the 4 character password on the right. Click DOWNLOAD. Wait for the time to elapse (lower right hand corner of table structure) and then download.