With the new version of any software there's always changes that leave us asking, "Where did that go?". With the introduction of the new Photoshop CS6, it was no different. With CS6 having more changes than any release of Photoshop in a very long time, you can be certain you will be wondering where some of your favorite tools went. Some were moved, changed or renamed but most of them still exist. It's just a matter of finding them. Lets take a look at where the Mask panel went and what happened to the Adjustments panel.
One change in Photoshop CS6 is the new Properties panel. In version CS5, the adjustments showed up in the Adjustments panel and we also had a separate Mask panel for modifying the mask edges. Now in CS6, the panel has been renamed to the Properties panel which shows us the controls for any of our adjustment layers like the Adjustments panel before. Now in the new Adjustments panel is the icons for selecting a new Adjustment layer which used to show up if you clicked on the big arrow at the bottom left of the old panel.
A few notable changes that also happened in this transition is the change of the icon to Clip the Adjustment to a Layer (circled towards bottom) and the addition of the Mask icon at the top of the panel. Because of the added Mask icon at the top of the panel they have eliminated the Mask panel altogether as its own separate panel. So if you're missing the Mask panel you now need to click on the Mask icon in the Properties panel to make it active. I know a lot of people have been looking for this so hopefully this answer one of the many questions of "Where did that go?" in CS6. You may also notice Adobe ditched the icon for making the panel bigger that looked like two folders at the bottom of the old panel. You can still adjust the size of the panel but you need to drag it by clicking on the bottom right corner of the panel.
You're not out of the woods yet though. So now you've found the Mask panel but you may get it grayed out which will surely now make you pull your hair out. I know because I did. To get the Mask panel active you need to make sure you have the Mask icon selected in the Layers panel as well. I know, seems like a lot of trouble but with the dual purpose of the Properties panel we need to now choose between the two for control for either the Adjustment or the Mask.
Notice in this screen capture above, once the Mask icon is selected in the Layers panel the Mask panel now becomes active and we can use it. You'll also notice, circled in red, the changed icon for adding a Vector mask.
I'm not a real big fan of this new functionality but it is what it is and you'll need to learn to deal with it if you use CS6. I personally miss the Mask panel because it allowed me to keep both open at once on my second monitor which I thought was very efficient.