Try creating a new Android project in your Eclipse IDE, then copying the hidden .classpath file from the new project into your existing one. You can then delete the extra Android project.
Then do the steps above. Delete the /gen folder, and do cleaning (Project -> Clean, Android Tools -> Fix Project Properties).
The issue for me was that the downloaded .classpath was set to place the source .class files into /gen, which was supposed to have R.class files. Eclipse wanted the source .class files in /bin, which is what Eclipse's automatically generated .classpath gives you. If you feel comfortable editing the .classpath yourself (it's not particularly difficult to read), you can make the modifications in-place without bothering to create a new Android project.