Take a look at the Working with Building Blocks item in the Word Developer
Reference that you can access through the Help facility in the Visual Basic
Editor.

It contains the following on inserting a Building Block Entry:

Inserting a Building Block into a Document

After you have access to a building block, use the Insert method of the
BuildingBlock object to insert it into a document. The following example
expands the previous code sample by adding a line for inserting the building
into the active document at the Insertion Point (or for replacing the
selected text, if text is selected).

Note

When you insert a building block by using the Ribbon, Word automatically
determines certain things about the building block, such as where to insert
it; however, when you insert a buildng block through the object model, none
of this built-in intelligence automatically happens. For example, when you
insert a header building block by using the Ribbon, Word automatically
determines to replace the existing header. When inserting the same header
building block by using the object model, you need to explicitly specify
where to place the building block text.

Sub InsertExistingBuildingBlock()

   Dim objTemplate As Template
   Dim objBB As BuildingBlock

   ' Set the template to store the building block
   Set objTemplate = ActiveDocument.AttachedTemplate

   ' Access the building block through the type and category
   Set objBB = objTemplate.BuildingBlockTypes(wdTypeCustomHeaders) _
       .Categories("Book Titles").BuildingBlocks("Title")

   ' Insert the building block into the document replacing any selected
text.
   objBB.Insert Selection.Range

End Sub