Microsoft Word Equation Editor is a cut-down version of MathType, by Design Science (http://www.dessci.com/en/products/mathtype/default.htm). MathType 6 for Windows works with Microsoft Office 2007, 2003, Office 2002 (XP) and Office 2000.
To access the Microsoft Word Equation Editor, it must be added to the installation of Microsoft Office at the install time, or added after install time using add/remove features in Office Setup. After installing Equation Editor, you can use the InsertEquation macro
Sub InsertEquation()
Selection.InLineShapes.AddOLEObject ClassType:="Equation", _
Filename:="", LinkToFile:=False, DisplayAsIcon:=False
End Sub
but that's only an interactive session. there's no way to generate an equation from a text representation other than using an EQ field, which is quite limited and you can't calculate. For more information, search "EQ field" in http://support.microsoft.com.
See also
http://support.microsoft.com/kb/228569
http://www.microsoft.com/education/insertequation.mspx
http://office.microsoft.com/en-us/word/HP051902471033.aspx