Missed that earlier somehow. Just regular sc#addWmpAre usage. It creates/looks for save and mpsave in the game folder rather than in .../User/Documents/BGEE/etc on WinXP.
Unfortunately, I don't think I am able to install BGEE on my WinXP system. But there are two potential points of failure in the code: either your game is not detected as BGEE, or WeiDU reads the wrong path from the registry. Does ADD_JOURNAL work on your BGEE game? (If so, it's detected as BGEE.) Can you also confirm that the actual value of %SAVE_DIRECTORY% is incorrect (e.g., by PRINTing the value)? Are you able to navigate the Windows registry and look up the value WeiDU is using? If so, it is "HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders"->"Personal" (sorry, I don't know what terminology is used when referring to the Windows registry). The value of "Personal" should be your Documents directory.