How can I troubleshoot why TinyMCE won’t load custom plugins in the visual editor?

You should read this: http://codex.wordpress.org/TinyMCE_Custom_Buttons

It’s pretty straightforward if you already know how to program a TinyMCE plugin (which it sounds like you do).

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)