To avoid further confusion please note that all Cubase LE or AI software packaged with hardware is OEM. The standalone software that we sales for Cubase begins with Elements 7 and go to Cubase Artist 8 and Pro 8.
The Steinberg installer Tool for Yosemite is not guaranteed to work for unsupported software but is only a hotfix for certain installers. Though LE6 is not supported on Yoemite, you can try to download it from the following full installer, which may work better than the disc installer that came with your N.I. audio interface.
This is a .ISO disk image you will need to mount the image in order to run the installer. The image should automatically mount on OSX operating systems.
Once the download is completed please use the Steinberg Application Installer Tool, to complete the installation on Yosemite.
*When running the installer make sure that all other applications are closed.