There are two ways you can access a category programmatically. You can either access it by index (where 0 is the top category):
var category = toolbox.getToolboxItems()[0];
Or by ID:
var category = toolbox.getToolboxItemById('categoryId');
Where the ID is specified in the toolbox definition:
JSON
{
"kind": "category",
"name": "...",
"toolboxitemid": "categoryId"
}
XML
<category name="..." toolboxitemid="categoryId"></category>