Dim Cat As ADOX.Catalog
Dim Tbl As ADOX.table
Dim Idxs As ADOX.Indexes
Dim Idx As ADOX.Index

Set Cat = New ADOX.Catalog
Cat.ActiveConnection = CurrentProject.Connection
Set Tbl = Cat.Tables("Имя таблицы")
Set Idxs = Tbl.Indexes
Set Idx = New ADOX.Index
Idx.Name = "Имя индекса"

Idx.PrimaryKey = True
Idx.Unique = True
Idx.Columns.Append "Имя столбца"
Idxs.Append Idx

Эээээхххххх, не туда уехал :)

Dim Cat As ADOX.Catalog
Dim Tbl As ADOX.table
Dim col As ADOX.Column

Set Cat = New ADOX.Catalog
Cat.ActiveConnection = CurrentProject.Connection
Set Tbl = Cat.Tables("Имя таблицы")
Set col = New ADOX.Column
With col
.Name = "Имя поля"
.Type = adInteger
Set .ParentCatalog = Cat
.Properties("AutoIncrement") = True
.Properties("Seed") = CLng(10)
.Properties("Increment") = CLng(10)
End With
Tbl.Columns.Append col

Забыл сказать
.Properties("Seed") = CLng(10) - 10 -стартовое значение
.Properties("Increment") = CLng(10) - 10 - шаг

кузя, надо ведь ParentCatalog указать