Long shot: Add keyword values and swap out spaces in the path with
%20.

Try:

INSERT INTO rpt_NewLoanMonitor VALUES SELECT * FROM [DataSource] IN 'C:
\Working%20Files\SQL_Scheduler.xls' 'Excel 8.0;'

Even if the syntax is OK the statement won't work if the number of
columns in rpt_NewLoanMonitor doesn't equal the number of columns in
SQL_Scheduler.xls.