Comment: Issues when posting articles

Well, actually you have to select a section (not a category) to post a new article.

In YACS any page is anchored to one, and only one, section, and to a variable number of categories.

More at What is the difference between a section and a category?

You have to create a page first, and then to link it to categories. While browsing the new page, click on the link Assign categories at the right panel. Then do your homework...

Thank to your message I have reviewed and corrected the page How to post an article? to make things more clear. I also have added the blogging by e-mail...
>is there any way to break the built-in forums down into categories?

Generally speaking, there is no way to split a section into categories. But you can create several sections and configure each of them as a dedicated discussion board
>how do I add tabs to the top of the site as seen here? Is this done within YACS or are they coded in?

The top tabs are supported by some YACS reference templates. To make a section appears as a tab, you have to change its ranking to less than 10000.

To do that browse the target section. Edit it. Expand the Advanced options. Change the Rank field from 10000 (which is the default) to 9000 (this is an example). Save and enjoy.

If you read some French this has been explained at Onglets de navigation