The Microsoft ODBC Data Source Administrator is used by DB2 to create an ODBC DSN. By default on a Windows 64-bit server, when the 'MicroSoft ODBC Data Source administrator' is run, it will launch the 64-bit version of odbcad32.exe, which can be used to create DSNs pointing to 64-bit IBM DB2 ODBC drivers only.
If you have 32-bit applications running on a 64-bit Windows server, they will need to use the 32-bit ODBC driver. To create DSNs pointing to 32-bit IBM DB2 ODBC driver, the 32-bit 'MicroSoft ODBC Data Source administrator' must be used.
Microsoft Windows 64-bit platforms maintain a compatibility feature which enables a 32-bit application to run transparently in a 64-bit environment, known as WOW64, (Windows-32-on-Windows-64). This maintains its own copy of the registry that is only for 32-bit applications.
This is also covered by the technical article 1384435, http://www.ibm.com/support/docview.wss?uid=swg21384435