Hello marcgh,

which editor provider you are using DropDownEditorProvider or DatePickerProvider? 

If you are using DropDownProvider you can use something like this:

    // Enable cell editing
        this.WebDataGrid1.Behaviors.CreateBehavior<EditingCore>();
        this.WebDataGrid1.Behaviors.EditingCore.Behaviors.CreateBehavior<CellEditing>();

        // Create an editor provider
        //DatePickerProvider datePickerProvider = new DatePickerProvider( );
        DropDownProvider dropDownProvider = new DropDownProvider();

        //Set ID and DataSource
        dropDownProvider.ID = "DropDownProvider1";
        dropDownProvider.EditorControl.DataSourceID = "AccessDataSourceCategories";

        dropDownProvider.EditorControl.DataKeyFields = "CategoryID";
        dropDownProvider.EditorControl.TextField = "CategoryName";
        dropDownProvider.EditorControl.ValueField = "CategoryID";

        // Add to collection
        //this.WebDataGrid1.EditorProviders.Add(datePickerProvider);
        this.WebDataGrid1.EditorProviders.Add(dropDownProvider);

        // Create a column setting to use the editor provider
        EditingColumnSetting columnSetting = new EditingColumnSetting();
        columnSetting.ColumnKey = "CategoryName";

        // Assign editor for column to use
        //columnSetting.EditorID = datePickerProvider.ID;
        columnSetting.EditorID = dropDownProvider.ID;

        // Add column setting
        this.WebDataGrid1.Behaviors.EditingCore.Behaviors.CellEditing.ColumnSettings.Add(columnSetting);

Check this samples to see how is implement client binding in WebDropDown:
http://samples.infragistics.com/aspnet/Samples/WebDropDown/Data/jQueryClientTemplates/Default.aspx?cn=drop-down&sid=1cd0d5b4-a6c5-45c2-835b-1d8b230dfb18

http://samples.infragistics.com/aspnet/Samples/WebDropDown/Data/BindingToHierarchicalJSON/Default.aspx?cn=drop-down&sid=c9dd40d1-0068-483e-88e0-d6fac427ba2a

Hope this helps