Drop Down in List Grid

Hi

I am using smartgwt 4.0
I want a drop down in a list box as shown in some examples in smartgwt client

I have created the following in the Datasource

DataSource categoryDS = new DataSource();

SelectItem selectItemFilter = new SelectItem();
selectItemFilter.setWidth("*");

DataSourceTextField category = new DataSourceTextField("category", "Category");
category.setEditorProperties(selectEditor);

categoryDS.setFields(category);

ListGrid categoryListGrid = new categoryListGrid();

categoryListGrid.setDataSource(categoryDS);

ListGridField category = new ListGridField("category", "category");

categoryListGrid.setFields(category);

When I try the following and click on the cell of listGrid I dont see the drop down
How can I create a drop down in List Grid

Regards
MJ