where can i find dbmsscheduler in plsql?

You can find DBMS_SCHEDULER jobs in the Jobs folder in recent versions of PL/SQL Developer. In older versions you’ll have to run select * from all_scheduler_jobs;

Your old version was probably created before DBMS_SCHEDULER existed, so your Jobs folder only reads from ALL_JOBS. Newer versions have two folders, DBMS_Jobs that reads from ALL_JOBS, and Jobs that reads from ALL_SCHEDULER_JOBS. When you find your scheduler job you can right-click on it and either view or edit the properties.

For example, if I create this job:

begin
    dbms_scheduler.create_job
    (
        job_name   => 'TEST_JOB',
        job_type   => 'PLSQL_BLOCK',
        job_action => 'begin null; end;',
        auto_drop  => false);
end;
/

I see these results in the Object window: