Once you're logged in, the grey floating 'Tool' menu at the right hand side of the page enables you to edit the page.
Like most Wikis, the site doesn't use HTML for formatting (security reasons etc). A basic guide to the syntax can be found here. Best rule - keep it simple.
Registered users can also navigate back to previous versions of the page to undo recent changes.
The best way to create a new page is to use the 'Start a New Page' link (above left) That way you can easily choose which category to create the new page in.
External links are recognised automatically, the easiest way is just to type or paste the link like this : http://www.example.com or simply www.example.com
Alternatively, a neater (but longer-winded) technique is to use the 'External Link' button in the editor toolbar - which will insert a dummy link which can then be adjusted with the correct url and text.
For internal links (i.e. links to existing pages within this wiki), highlight the relevant text and use the 'Internal Link' button in the editor toolbar, which will allow you to pick the internal page you wish to link to.
Again, a basic guide to the syntax can be found here.
If you'd like to experiment, use the 'sandbox' pages, where you can create anything as a test.
In a departure from normal 'wiki' pages, it's possible to add a 'rating' for the content (it appears at the top of the page). The idea is to distinguish exceptionally enigmatic and important problems from minor ones. (see top of this page as an example)