Eikon_Install_Folder\Bin\Apps\TR.OFFICE.CORE\0.0.0.0\Bin
E.g. if current version of Eikon is installed in C:\Program Files (x86)\Thomson Reuters\Eikon\X\, then the full path is C:\Program Files (x86)\Thomson Reuters\Eikon\X\Bin\Apps\TR.OFFICE.CORE\0.0.0.0\Bin\Interop.Dex2.dll