There are 2 methods you need to use to sync FlexCelGrid and FlexCelImport.
1) When you change things in FlexCelImport and you want them applied to FlexCelGrid, you need to call:
FlexCelGrid.LoadSheet;
2)When the user changes things in FlexCelGrid and you want them changed in the FlexCelImport, you need to call:
FlexCelGrid.ApplySheet;
While most things are synchronized automatically, it is a good idea to always call those methods whenever you change somehting in the grid/FlexCelImport so it is reflected in the other.
You can look at the XlsViewer demo and search for "LoadSheet" and "ApplySheet" to have an example of how they are used.