You have not given your RDBMS version or platform so I will assume 8i on Unix.
Code:
Logon the SQL*PLUS as USER SYS
SQL> CONNECT sys/password
Execute the plustrce.sql script
SQL> @$ORACLE_HOME/sqlplus/admin/plustrce.sql
Grant Role PLUSTRACE to User
SQL> GRANT plustrace TO username;
Logon as the user requiring the usage of AUTOTRACE
SQL> CONNECT username/password
Create an explain plan table, Execute the utlxplan.sql script
SQL> @$ORACLE_HOME/rdbms/admin/utlxplan.sql
This should solve your problem.
regards