The "Data Sources (ODBC)" application, by default, creates System DSN names that are created in the 64bit part of the Windows registry, and as such are only visible and accessible to 64-bit applications. Since EventSentry only ships 32-bit applications, you will need to create a System DSN using the 32-bit version of the "Data Sources (ODBC)" application.
To create a 32-bit System DSN, click "Start - Run" and enter the following path:
(If you installed Windows in a directory other than C:\Windows then you will have to adjust the path accordingly.)
Any System DSNs you enter now will be visible only to 32-bit applications and should work with EventSentry.