Huh? Client should always try to find in data/ folder, or whatever is set; make sure that you have correct value of FonlineDataPath in FOnline.cfg ("./" / ".\" prefix may be important, even on windows)
As for loading, try something like ["dir/file.frm", PT_ART] or ["art/dir/file.frm", PT_DATA] or in worst case ["data/art/dir/file.frm", PT_ROOT]