Let me start by saying that I have checked the other questions that this could be considered a "duplicate" of and none of them solved the problem for me.
When I open an Android layout file in Eclipse 4.2, I am unable to open it in the Layout Designer, only in the XML text editor.
There are several similar questions that have been solved, the accepted solutions were to use Open With, or restart eclipse. I have tried both of these several times but the Android Layout Designer is not in my "Open With" dialog. There is a Legacy Android Layout Editor but it also does not display the Layout Designer.
I would have posted an image if I had enough reputation.
Possibly Android ADT plugin not developed properly for Eclipse 4.2 yet, You can switch back to Eclipse Helios with ADT-19 or ADT-20 plugin.