Get/set value of "Multiple category selector" form control field.

the multiple category selector sets the documents categories, it does not save the category name's in the field.

you can use my advanced category selector on the market place and set the save mode to "field" or "field and categories" since this will save the category name/guid/id to the field, or you can look at the documents categories using the api. I think the treenode object had a categories property you can loop through, otherwise just use either the categoryinfoprovider class

I second Trevor. Multiple categories selector is just a different way of assigning categories. If you go to document properties/categories and assign categories there you will achieve exactly the same. if you have a tree node you can use API to add/remove categories

// Gets the category
CategoryInfo category = CategoryInfoProvider.GetCategoryInfo("Dogs", SiteContext.CurrentSiteName);

if (category != null)
{
    // Gets a page
   var treeNode = DocumentHelper.GetDocuments("classname")
            .WhereEquals("NodeID", 123)
            .OnSite(1)
            .FirstObject;

    // Adds the page to the category
    DocumentCategoryInfoProvider.AddDocumentToCategory(treeNode.DocumentID, category.CategoryID);
}

P.S. Don't forget that categories get assigned per document (not per node).So if have different cultures on your site like French and English then for a given node you might have 2 documents: English and French. If you assign categories to English they don't get assigned to French.