I also was not able to get Time Machine to work by selecting the top level of "Macintosh HD"; I found instead I could only access my old backup by selecting the top level of my computer (not any particular volume).
To be clear my situation was the following (perhaps a bit different from others in this thread): I had a Time Machine drive for an iMac running Mavericks and I used migration assistant to transfer my data to a new iMac running Yosemite. I then selected "Inherit backup history" when I did the first backup of the new iMac to the Time Machine drive.
When I attempted to access old backup the usual way (i.e. entering Time Machine from some subdirectory of my home directory) I could not access my old backups. I was also not able to access my old backups from the top level of "Macintosh HD" as people had suggested above. However, I had forgotten that I had renamed my internal HD on my old iMac, whereas my new iMac was still named "Macintosh HD" (I'm not sure if this was related to my inability to access backups from "Macintosh HD"). However, when I moved to the top level of my iMac (not a volume), I could access my old backups and navigate down the directory structure.
Am I correct in thinking the confusion (in my case at least) is that Time Machine seems to be organizing the backups in a way that keeps a distinction between the two different drives (either different name or different physical device), and thus won't let you browse back in time from inside the "new" volume, but will let you browse top down from the old backups?