If you are using the RT edition of Windows 8, such as what is currently sold on Surface tablets, do note these are not designed to run "legacy" Windows applications and are mostly designed with a view to utilising Windows 8 apps, so software such as Pokémon World Online will not run at this time.
We are not aware of issues running PWO on other versions of Windows 8, so if you do not believe the above to be the case here, please explain the problem you have in more detail.