OpenSTAADOEM is installed along with STAAD.Pro SS6 version. You don't need to install it separately. OpenSTAAD.dll files were supplied previously which were located in the OpenSTAAD folder. Since STAAD.Pro SS6 version, those .dll files have been replaced with the .tlb files. You can find the OpenSTAADUI.tlb file in the following location--
X:\SProV8i SS6\STAAD
where X is the local drive where you have installed STAAD.Pro SS6 version.
X:\SProV8i SS6\STAAD
where X is the local drive where you have installed STAAD.Pro SS6 version.