CASCExplorer is using a LISTFILE to read the files from the World of Warcraft DATA. If you cannot find the WMO's, then it's most probably because your LISTFILE is outdated. Take a look into your CASCExplorer folder, you will find a file called LISTFILE.TXT - open it and do a search.
If you cannot find it, then you have to add the full path into the LISTFILE.
As an example: WorldWMODungeonRoyalQuartersPA_RoyalQuarters.WMO