How do I add a new page?
If your new page will be referenced from an existing article, edit that article and add a link to where you want your new page to go. For example, to add a link to mdconfig from the article Mounting ISOs under FreeBSD 5.x, you would insert the text [[mdconfig]] in that article, then click the link after submitting your edit - if there isn't already an article for mdconfig, you will be taken to a blank article you can then edit.
If your new page is, well, new - then the easiest way to get there is to just add the name of your proposed article to the URL in the address bar. For example, to create an article on long-haired mountain goats, you could type http://freebsdwiki.net/index.php/long-haired_mountain_goats into your address bar. The wiki will then take you to a blank article and let you get started.
It's a good thing to check for existing pages so you don't reinvent the wheel from scratch, but if you do you can edit the pages into being one and delete the redundant one (assuming the two pages are completely redundant of each other).
 How do I add my article to one of the category pages?
Just add the text [[Category : NameOfCategoryYouWantToBeIn]] to the end of your article. Presto, it's added! If the Category you specify doesn't exist, it will be created. If it does exist, you'll be added into it.
 How do I create a new category?
Just like creating a new page, you can either create a tag adding an article to your new category, or you can create the category by editing the URL in your address bar, in the form http://bsdwiki.tehinterweb.net/index.php/Category:YourCategoryName.