//Create a new search
Search s = new Search();

//set the selection to everything
s.Selection.SelectAll();

//Add a search condition
s.SearchConditions.Add(SearchCondition.HasCategoryByName(PropertyCategoryNames.Geometry));

try
{
   //get the resulting collection by applying this search
   ModelItemCollection searchResults = s.FindAll(Autodesk.Navisworks.Api.Application.ActiveDocument, true);

   //Select the items in the model that are contained in the collection
   Autodesk.Navisworks.Api.Application.ActiveDocument.CurrentSelection.CopyFrom(searchResults);
}
catch (CanceledOperationException)
{
   //the user cancelled the search
}