On edit page: where is the plugin tab?

This depends on whether you are using the "Classic" text-based editor, or the newer WYSIWYG (what you see is what you get) point-and-click editor.

The plug-in command is only available in the point-and-click editor. Don't worry, if you're like me, and you prefer using the classic editor, you can always switch back later.

From the classic edit page, click the "Point-and-Click Mode" link in the upper right corner of the window. This will switch you to Point-and-Click Mode.

From there, the "Insert Plugin" command will be on the far right-hand side of the first row of edit commands, right above "Attach File".

When you're done, save the page. Then if you want to switch back to classic mode, click "Edit" once more, and click "Switch to Classic Mode", which will be in the upper right corner.