Since your drop down list is inside the repeater, make sure you reference the DataItem
to find the control.
Make sure to use e.Item.FindControl
rather than Page.FindControl
-- Page.FindControl
will not find this item because it will not recursively search the page
protected void rptActivity_ItemCreated(object sender, RepeaterItemEventArgs e)
{
var helper = (UpdateActivitiesHelper)e.Item.DataItem;
DropDownList ddl = (DropDownList)e.Item.FindControl("ddlStatuses");
ddl.SelectedValue = helper.StatusId.ToString();
}