How exactly did you set it? You do not do it in preferences (a common mistake). You do it by starting up iTunes while holding down the option/alt key whereupon iTunes will irreversibly convert what it sees as the library, namely the iTunes Library.itl file, to a version which will only work with the newest version of iTunes you run (your 12). Clearly you can't use 12 on your old computer so there is no way to share the same library file. Each version of iTunes will have to run its own library file which will essentially maintain its own independent list of entries, though they can share media. This means changes you make to one library will mostly not appear in the other but if you do something such as delete media in one library the other library will fuss it can no longer find the file, etc.
If you are going to start doing advanced iTunes things you need to learn how iTunes works.
What are the iTunes library files? - http://support.apple.com/kb/HT1660
More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management
What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.html
Where are my iTunes files located? - http://support.apple.com/kb/ht1391
iTunes 9 [and later]: Understanding iTunes Media Organization - http://support.apple.com/kb/ht3847 - plus supplemental information about organizing to new structure https://discussions.apple.com/thread/6477809?answerId=26404702022#26404702022
Image of folder structure and explanation of different iTunes versions (turingtest2 post) - https://discussions.apple.com/docs/DOC-7392 and making an iTunes library portable.
One other tip. Just about every mention I see of people using iTunes with a NAS is something with a problem that results from using iTunes with a NAS. iTunes is likely not written with NAS use in mind. It may work, or it may not. Keep regular backups.
