1. After you set the db_create_file_dest parameter using an ALTER SESSION command, use SHOW PARAMETERS DB_CREATE and see what the new value is. Try copy/pasting the value from there to another command that creates a file, and see if it work.
2. The directory you specify must already exist, Oracle will not create it.
3. On Unix only ... the names are case sensitive. You must set this parameter using the same case as the directory name at the operating system level.
4. You must have operating system permissions on the directory in which you want to create files.
5. If you have access to the on-line Oracle docs, then check out /server.920/a96521/omf.htm