you need client access installed. And the use ODBC administration to config an ODBC connection to the 400.
in Excel use the data ribbon, from other sources, from data connection wizard. In the wizard, select ODBC DSN as the data source. Click next. On this next screen you should see your as400 ODBC data source listed. Click the as400 data source, click next and you get a list of tables and views in the current library.
Go thru that and you should see how an entire table or view can be read from the as400 into excel.
More can be done with this, including calling stored procedures on the 400 that return a result set to excel.
-Steve