The 32-bit version of the ODBC Administrator tool and the 64-bit version of the ODBC Administrator tool display both the 32-bit user DSNs and the 64-bit user DSNs in a 64-bit version of the Windows operating system

http://support.microsoft.com/kb/942976/en-us

To set up a 32-bit DSN launch the 32-bit version of the Data Source Administrator. It is located at:
%systemdrive%\Windows\SysWoW64\Odbcad32.exe


If this step is not carried while creating a ODBC connection the vcenter installer will give you a message.

(Please create a 32 bit system DSN)

Upgrade to vCenter Server 4.0 fails with the error: DSN is pointing to a unsupported ODBC driver.

http://kb.vmware.com/kb/1015804

Note: Please ensure you select the SQL Server Native Client 10.0 while creating the DSN.


HTH

enjoy :)