It was asked a couple of times in our forums how to create masking in Flash. Its pretty basic, I always try to explaing with images so that its more illustrated and easy to understand, I hope this will help anyone :)
1. Create new file.
2. Insert new layer by right clicking the "layer1".

3. Write TEXT in the top layer "layer2", text is "SWiSH-DB is the largest community of developers".
4. Click on "layer1" and draw a circle (which will be the mask)..
4. Right click on "layer2" where we added text and click on
Mask
Well - thats all, if you cannot get it working, please post in forums or amy be post comments here.