The Wiki-Structure is shown in the left wiki-navigation. To see the navigation one has to scroll down, cos there is the navigation-area for "Subcommunities" before (when having a lot of sub-communites, the wiki-navigation will get invisible). Why not moving the Wiki-Navigation directly under the Mainnavigation-Item "Wiki" (see screenshot for example)?
Additionally, when clicking in the wiki-subnavigation, often the page-content is loaded via ajax call and therefore the browsers stays at its current scroll-position - so the new content is displayed somwhere above the visible area. Solution could be: when clicking in the left sub-navigation to load anothers wikipage content there should be an auto-scrolling back to the top of the page (this is nothing new for webdevelopment).