Hi Alex,
In Excel VBA first reference the Inventor Object Library:

Then use GetObject or CreateObject to connect to a running session of Inventor or create a new session. This post has an example:
http://forums.autodesk.com/t5/inventor-customization/getobject-need-help/m-p/759841/highlight/true#M...
Once you have the Inventor Application object you can use the Open method of the Documents collection. Below is an Inventor VBA example from the help file. It shows how to open an assembly with a level of detail. You could just change it and pass in the path to your assembly.
Public Sub OpenDocumentInLastActiveLOD()
Dim strFullFileName As String
strFullFileName = "C:\temp\Assembly1.iam"
' Set a reference to the FileManager object.
Dim oFileManager As FileManager
Set oFileManager = ThisApplication.FileManager
' Get the name of the last active Level of Detail (LOD) Representation.
Dim strLastActiveLOD As String
strLastActiveLOD = oFileManager.GetLastActiveLevelOfDetailRepresentation(strFullFileName)
' Use the full file name and LOD name to get the full document name.
Dim strFullDocumentName As String
strFullDocumentName = oFileManager.GetFullDocumentName(strFullFileName, strLastActiveLOD)
' Open the document in the last active LOD.
Dim oDoc As AssemblyDocument
Set oDoc = ThisApplication.Documents.Open(strFullDocumentName)
End Sub
Thanks,
Wayne
Wayne Brill
Developer Technical Services
Autodesk Developer Network