If you cannot find the virtual account used by the default settings installation of your SQL 2012 instance (in this case running on Windows 7), try searching for:
nt service\mssql
You must have the "Built-in security principals" object type selected, and you must be searching on your local machine name (not in the domain).
Frustratingly, searching for the virtual account does not work for these strings:
- MSSQL$SQL2012
- MSSQL
- nt service
- nt service\
- NT
Which is misleading...