in a sql worksheet try some of these:

ctrl-shift-0..9

This will pop the 0-9th sql form the history and replace the current sql worksheet contents.

The rest need to be run as if they are scripts ( F5 ):

Want to change the tab in the worksheet while a script runs to see an easy status.
   set worksheetname hello
Working on APEX or mod_plsql routines?
   set owacgienv on
   set getpage on
   begin
      htp.p('hi');
   end;
   /
This will make the owa.init_cgi_env before a plsql block and owa.get_page after and print in the script output.


Lastly, if you have any issue you can turn debug on with this and then take it to the forums to post and get answers:

setloglevel oracle INFO