I use PS a lot but it's very confusing and I'm sure someone else could give you a better or more definitive answer.
If I'm understanding your question right, you should be able to select "Edit", "Transform or Transform Path", "Scale" and you should see the "X" and "Y" pixels to modify.
I work with PS7 so it may be different in your version. I've also noticed that sometimes it depends on whether the layer is rasterized or not for "Transform" to be selectable.
