Tdyn can be extended by using the Tcl script language. Tcl, or the "Tool Command Language", is a very simple, open-source-licensed programming language. Tcl provides basic language features such as variables, procedures, and control, and it runs on almost any modern OS, such as Unix, MacOS and Windows computers. But the key feature of Tcl is its extensibility.
You may find further information on Tcl at http://wiki.tcl.tk/969.