I opened After Effects CS5 added an Opacity keyframe, when to change the value the box was gone. Normally it's located on the Sublayer menu directly under where it says None for each layer. This is what I it looks like
In the very bottom left corner there is a button that looks like some files, it's the "Expand or Collapse the Layer Switches" toggle. If you press that, it should restore your value box. If not right click in the area above your layer next to source name and you can choose "Columns -> Switches" to turn them back on.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.