iframe code

I uploaded additional-tags to mu-plugins folder. From the instruction, "add additional tags (embed, iframe, etc) to be used when writing a post or page (in their html tab)." I tried to embed the several iframe codes in the post with HTML, but it doesn't show up. All the code were gone when I switched HTML to visual. However, if I put the same code in the widget, it works.

I am using WP MU 2.9.1.1 and buddypress 1.1.3. The theme I am using is buddypress social theme. Is it possible to fix it?
Thanks.