Call transaction I18N
go to I18N Customizing -> MS Word as Editor.Choose the Help-Button and follow the instructions.
If transaction I18N is not available in your system yet (see SAP Note 848036), you can also enable this feature via program RSCPINST.
Call transaction SE38, enter program RSCPINST and press F8. On the next screen,select Goto -> Set MS Word as Editor.