I had this problem too on my CS5 version. The little underlined field next to the switch for entering percentages for the various layer values (opacity, rotate transform, et al) had been there and I was using it. Then during the course of juggling other items when I went back to try to enter values – the little underlined field was gone…and it still is.
I’m not sure what I did to make it go away but there is another way to enter the values.
Make sure the correct layer you want is selected, twirl down the triangle to reveal the layer value names, Control click on the value name, and a sub menu comes up, click on “edit value”. A floating mini window that contains a boxed field appears. Enter values there.