When installing vSphere on a 64bit Windows host in x86 compatibility mode, a 32bit Data source name (DSN) is required for the database connection. If a DSN has been set up through Start > Administrative Tools > Data Sources (ODBC), the vSphere installer cannot detect it.
 
The 64bit ODBC Administrator tool can be used to manage user DSNs and system DSNs that are used by 64bit processes. For 32bit datasources, the 32bit ODBC Administrator tool is used for Windows on Windows 64 (WOW64) processes.
 
To set up a 32bit DSN, launch the 32bit version of the Data Source Administrator. It is located at %systemdrive%\Windows\SysWoW64\Odbcad32.exe.

For Microsoft SQL Server database servers, install the SQL Native Client from Microsoft Server media or Microsoft download. For more information, see the Configure a SQL Server ODBC Connection section in the ESX Server Installation Guide.
For Oracle database servers, install the 32bit database ODBC drivers. For more information about setting up an Oracle connection, see the Configure Oracle Databases section in the ESX Server Installation Guide.
The vSphere installer can now detect the DSN and continue installation.