Hi all.  I'm pretty new to Photoshop CS5 (I've only done some basic selective coloring, ect) and I wanted to try to make some harder icons, but when I try to use the mask tool I can't make it work.  I'm sure I'm just making some sort of basic n00b mistake but I click the top layer, click the little circle inside the square at the bottom to (presumeably) to create a mask, click the brush tool, and then I'm stuck.  Either it merely paints on my pictures or it shows no brush changes at all.  In case that doesn't make sense, the point is so that I can make icons made of two screenshots like those in this tutorial:  http://community.livejournal.com/icon_tutorial/11197506.html#cutid1  If anyone can help me at all I would really appreciate it.