For a 64-bit Windows 7 system, when you run the ODBC Data Source Administrator tool (odbcad32.exe), you run (by default) the 64-bit version of this tool. It does not support the 32-bit system data sources.
To see the 32-bit system ODBC drivers, you must execute the 32-bit version of the ODBC Data Source Administrator tool. In a typical Windows7 installation, it will reside here:
C:\Windows\SysWOW64\odbcad32.exe
Here is the URL to a complete explanation: