if you get error REP-3000: Internal error starting Oracle Toolkit.

if you get this error, first check your host with oracle user, if you can open xclock. If you can't, run this under root privilledge,
xhost +

but if you can, but still having the same error, follow these instruction.

To manually add the DISPLAY value:

1. Change directory to $ORACLE_HOME/opmn/conf.

2. Back up the opmn.xml file.

3. Add a new line of type in the opmn.xml file configuration section for the new Reports server. Use the following example as a guide, where reportserver_name is the name of the Reports server, and display_value is that value of the DISPLAY

        
   ..process-type id="ReportsServer" module-id="ReportsServices"..
     ..process-set id="reportserver_name" restart-on-death="true" numprocs="1"..
        ..environment..
           ..variable id="DISPLAY" value="display_value"/..
           ..variable id="PATH" value="/project1/AS10g/FR_STANDALONE_M23_MAR08/ohome/bin:/usr/bin:/etc:/usr/sbin:/usr/ucb:/usr/bin/X11:/sbin:/usr/java130/jre/bin:/usr/java130/bin:/usr/bin:/bin"/..
   
   
   

4. Reload the opmn.xml file, as follows:

   $ $ORACLE_HOME/opmn/bin/opmnctl reload
   
   

5. Stop and then restart the Reports server:

   $ $ORACLE_HOME/opmn/bin/opmnctl stopproc ias-component=reportserver_name
   $ $ORACLE_HOME/opmn/bin/opmnctl startproc ias-component=reportserver_name
   
   

6. Verify that you can run a test report. For example:

   http://mydomain.com:7782/reports/rwservlet?destype=cache&desformat=html&server=reportserver_name&report=test.rdf&userid=scott/tiger@twotask

source : http://download.oracle.com/docs/cd/B13597_05/bf.904/b13667/toc.htm