I know how to do it in Illustrator, but the same procedure doesn't work in InDesign, so either it can't be done, or there is are different steps involved. I would guess that Adobe are presuming you have created all your elements in Illustrator/Photoshop, and using InDesign to just lay then out. After all, that is what InDesign is for. If InDesign could do everything Illustrator could, there'd be no reason to make both applications.
If you have Illustrator, you can do it there and export it accross to InDesign. That way it will still be vector based so you can still resize it with no quality loss.
Opacity Masks in Illustrator
1. Create an object you want to apply the trasparency to (ie. a box with a solid fill colour, or a group of objects, whatever. If you use more than one object, make sure it is grouped).
2. Create a new object over the top of the first, and apply a black and white gradient fill to it. This is done by creating the object, then bringing up the gradient palette (Window > Gradient). Then, with the object still selected, click in the gradient fill image inside the Gradient palette.
You will use this gradient object to determine which parts of the first object are transparent, and which are not.
3. Select the gradient object, then Shift-click the other object, selecting them both at the same time. Or just drag a select box around them and select them that way. It doesn't really matter, so long as you have two objects selected, and the object on the top is the gradient fill object.
4. Bring upthe Transparency Palette (Window > Transparency). Click the fly-out menu of the transparency palette by clicking the small arrow button at the top-right, and choose "Make opacity mask".
5. Wipe away your tears as you gaze upon the glory of your first opacity mask
Frankly, it is a little over-complicated for simple masks like this. Used more complexly, the opacity mask is friggin amazing, but I urge Adobe to include a more easy to use (and find) tool for simple transparency gradients. I must admit Corel Draw's transparency tool is a lot better than Illustrators for this sort of thing (but unlike Illustrator's, Corel's tool was very limited in what it could achieve).