Hopefully, using the Charity Engine Wiki is easy. Here are a few things that might not be obvious, though...
The most common way to create a new wiki article (which will have its own web page) is to add a link to it from an existing article, eg. the title page. The new page will then be created, with the title you give it, empty and ready for content.
To add a link;
When editing an article, if you select text, you'll see that the "Insert link" icon becomes available in the edit window toolbar. Click on that and you'll see the "Insert Link" popup window. This window allows insertion of three kinds of links:
If you select the "Webpage" link option, then the popup allows you to provide any web address. When you click "insert", this address will be the "link to" address for the highlighted text.
If you select the "Wiki" link option then the popup expects an article title (for instance, the title of this article is "Using the Wiki"). When you click insert, the specified article will be the "link to" address for the highligted text. (Note: if there is no article with the provided title, one will be created. It is the easiest way to create a new article.)
If you're not quite sure about things, and you want to play around but don't want to mess up any "real" pages, try the Sandbox Page, which is there expressly for the purpose of trying things out. Add, edit, delete, whatever you like.