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If you push M the mask properties should appear - the one you want is "Mask Path"
If you click the stopwatch icon in front of Mask Path, then any changes to the shape of the mask will be keyframed on the timeline. Using the selection tool, if you hover over the control points, it can be moved where ever you want.
If you just want to change the size of the mask, there is a mask property available from the twirldown menu under Masks called "Mask Expansion" Increasing that value will cause the mask to grow outside the current bounds. Reducing it will contract the mask. This property also has a stopwatch and can be fully animated.
This property can be exported out of After Effects if you render out to a format that includes alpha values. In the Render Queue, the Output Module has a drop down that allows you to choose RGB+Alpha - that will get the transparency you are looking for. Not all formats support this, you may have to try a few different types. On a PC, you can use AVI with the Lagarith codec, just make sure you go into the codec settings and choose RGBA (RGB + Alpha) On a Mac, QuickTime Animation is a good choice.
Wow, thanks a bunch.
How does one make the Mask appear halfway through the project?
Even easier hold down the Ctrl/Cmnd key and press M. This will set a keyframe at the CTI for the mask property. The same trick, pressing Ctrl/Cmnd + s or a or r or t or p will set a keyfreme at the CTI* for Scale, Anchor Point, Rotation, Opactiy, or Position.
You could also turn on automatic keyframing.
This will set a keyframe any time you change any animatable property. Some folks like this. It drives me nuts, but it is another option.
Please, do follow the link that Dave gave. Animating a mask is covered in the first resource linked to from there.
It is very important to learn the basics.