I also found this in another post:
It may be as simple as you need to turn off the wysiwyg editor:
admin -> Users -> scroll to bottom left.
Also go to :
admin -> Options -> Writing and uncheck box for “WordPress should correct invalidly nested XHTML automatically”.
You could try using a browser that does not support WISYWYG editing. This shows only the code tab. (At least Mac OS Safari works this way.)
