Unidac, as you have discovered provides an ODBC data access driver. But as with other drivers that provide access via ODBC, you need to setup an ODBC data source name (DSN) to connect to via Control Panel->Administrative Tools->Data Sources (ODBC). And that's just the start...
A better option for Delphi is to go via ADO or... OLE.
In the Delphi IDE select Component->Install Packages, and tick either
Microsoft Office 2000 Sample Automation Server Wrapper Components
or
Microsoft Office XP Sample Automation Server Wrapper Components
to install the components you can use for accessing Excel this way.
Here is a comprehensive guide to OLE, though it's a little dated: Delphi 3 - Delphi and Microsoft Office: Automating Excel and Word and
Delphi 3 - Delphi and Microsoft Office: Automating Excel and Word - Page 2