I had the same problem with my old Singapore N9. (this was on pr 1.2). You first need to have nano installed. With that installed log in as root in terminal (devel-su pass:rootme) go to the sources.list.d directory and type
nano /etc/apt/sources.list.d/aegis-ssu-keyring-009.list This will open up Nano and you can edit this first repository line to say .../harmattan/001. / Save this and then run apt-get update and you should be able to install apps. May not work on pr1.3 but this is the workaround I used. You can then change the file back to 009 after to ensure all updates are available