This is accomplished in Mendix using CSS. At it's most basic level, this can be done by adding inline styles to your data grid. Look for the style property under the Common tab.
In that style property, try adding this:
I believe this should set the data grid's width to 2000 pixels (you can make this higher or lower as you need),
Next, add a Container widget, and put your datagrid inside of it. Add this style to that container:
That will force a horizontal scrollbar to appear around the grid. Your search and control button pane will scroll horizontally as well, but you would need to do some more in-depth CSS to keep them locked in place.
For a more in-depth discussion on styling your Mendix app, see the resources here:
AFAIK this isn't possible. You could try using the Datagrid Extension widget from the App Store for more controlability of your grid (if the widget is still Mx6 compatible).