I'm using Windows 7 and installed the 64 bit version of Eclipse 3.5.2. I then installed the Android ADT plugin, but when I try to configure it in the Windows > Preferences dialog, the Android Plugin doesn't show up in the left pane. Instead I see DDMS. This prevents me from specifying the location of the Android SDK (unless there is another way) to give me the appropriate templates and such.
Someone posted a fix to this that includes setting the permissions of Eclipse, but that didn't work for me. I tried installing the Android Plugin from both online installation (thru the URL install) and the offline Archive method.
If you're running Windows Vista or 7, make sure you right-click Eclipse and RUN AS ADMINISTRATOR. I literally spent six hours figuring this out, and this was what fixed it.