If you are using the text editor when you post (the editor provides boxes you click on to bold, italicise, etc), manually-entered HTML tags WON'T work. That's what the buttons are for.

In order to use HTML tags, you'll need to disable the editor. You can do this by clicking on My Account --> Edit --> Untick "Enable Rich Text Editor" --> Submit. This will allow you to enter your own tags (< em >, < blockquote >, etc)