I was trying to follow some instructions today, and it starts with the comment
REM In SQLPlus I manually copy in each line and execute it.
SQL Developer these days comes with another tool called sqlcl. This is a bit like SQLPlus but is actually using some bits from SQL Developer to give a compatible command line/scripting type interface.
You would be able to use it to execute sqlplus style commands without fighting the extras of the SQL Developer style GUI which can get confusing.
Look for it under wherever SQL Developer is sitting. If you don't have it there, you can download it and deploy it into your sqldeveloper folder.