Have you tried exporting an OMF file in Nuendo and opening that in Cubase? If this isn't possible, you are either going to have to open the wave files that Nuendo created and put them in a Cubase project manually or export track by track in Nuendo and open the resulting files in a Cubase project.

Both programs should offer the ability to select some form of a file format. And since both are from the same source, that should'nt be a problem. I'm currently away from my workstation, so I'll do the best I can from memory. When you select to open a song, go to: file> import cubase song> go to the source drive and select that file or folder. When that folder opens, select the appropiate file then go to the base of that window. You should have the option of selecting a program & file type. If both programs have use of the same base song/project format (which they should), you're in. But most of all, it will depend on how the original song(s) were saved (nuendo/cubase format that is). If that fails, you still should be able to import the files as .wav for each individual track.