Sounds like you are using the Visual side of the editor.
You need to use the Text side.
Get it?
Update:
Preview won’t show embedded objects. Publish it privately if you absolutely must see it before anyone else, and need to see embedded bits like <object> tags.
Update #2:
Preview will show many embedded objects now as of WP 4.0. And many embeds can occur from merely a URL (tweets, youtube, for example).