You can set the default path in EM. Right click on the server name and choose properties, then go to the database settings tab.

For an indirect answer to your question and some more information about the underlying keys, see:
272705 INF: Default Data and Log Directory Display Blank by Default After You
http://support.microsoft.com/?id=272705

Cindy Gross, MCDBA, MCSE
http://cindygross.tripod.com
This posting is provided "AS IS" with no warranties, and confers no rights.