lovelylovely treats the terminology too freely...
If you have a website hosted - the host usually offers some kind of site administration panel, like cPanel, Plesk etc.
This is NOT the same as your (WP) blog's admin panel! And the "Presentation" menu is in the WP admin panel, not in the site admin panel.
So, login to your blog > and you will see the Presentation menu.
Now, if you seriously want to get into the theme design business... you should forget the online editor, just design your theme offline, upload it, select it in the Presentation and you are good to go.
See also the possibility of installing it locally for design purposes: http://codex.wordpress.org/Getting_Started_with_WordPress#Installation