Instead of using a Datagrid you should use datagrid 2. Datagrid (1) does not have the option to add buttons, datagrid 2 does.
Datagrid 2 is only available in the newer versions of mendix by default. You might be able to download it from the marketplace. See documentation about Datagrid 2: https://docs.mendix.com/appstore/modules/data-grid-2
If you are still working in an older version of Mendix, you could also use a Listview instead of a datagrid, because buttons can also be placed there.
Did you place the datagrid in a Layout grid?
I imported the table with ExcelImport, there is no selection for data grid 1 or 2