PL/SQL Developer is a completely different product. You'll need to go their forums for help.
The ORA-942 error is telling you one of two things. Either the table does not exist or you do not have permissions to interact with it. In the case of the former, make sure you have spelled the table name correctly. If another user owns the table, you will have to prepend the table name with schema-dot notation. In the case of the latter, work with your DBA to see if you have appropriate permissions.