Well, look it up in the .NET API docs. Full name of the method is:
Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument.SendStringToExecute(..)
You pass it a string containing the command and params and some booleans, look it up. For example.
SendStringToExecute("select\nr\nall", false, false, false)
will deselect all selected objects in the active document, you separate the command and the params with line break symbols.
Janko Jerinic