What results do you see when you attempt this? Do you at least see the usual response from Design:
11/13/2008 12:52:27 Database Designer started
11/13/2008 12:52:28 Database Designer shutdown
Finally, are you saying that this works for one level of subdirectory, but fails for two levels or more? (For example, if you had your data directory, then /x/y. Issuing a LOAD DESIGN -d /x works, but LOAD DESIGN -d /x/y does not?)