You can find the html of the autonav block in;
concrete/blocks/autonav/view.php

copy that one and paste it into;
blocks/autonav/yourname.php
and make your edit's in this version such as adding your id's and classes.

If you now click on the menu block (in edit mode) you can select a Custom Template which is a list of the php files in blocks/autonav/
The custom template (which your classes and id's) should now be displayed as the menu.

You can create multiple templates, all place them in blocks/autonav/ . If you are using a globalblock as your menu, you'll also notice the option 'set custom template' which acts exactly the same way as described above.

Never edit the view.php in concrete/blocks/autonav since these might get overwritten on a systemupdate.

The above way of templating goes for all blocks that are available, just make sure you set them in blocks/BLOCKNAME/ instead of conrete/blocks/BLOCKNAME/

-Robert

Finally! I've been trying to figure out why I could not style the auto-nav. I see here that I have to create it.

Now, I want to point the one I made to my sites css. I don't see in the auto-nav.php file where I do that.

Any ideas? Thanks.