Sorry for the inconvenience. Known Navigation header breakage, its being "rebuilt" but forward nav should be ok now.
There may be other unforeseen breakage primarily in the handbook portion of the wiki.
The Table Of Contents section of each "PC-BSD® Users Handbook" page may be the best workaround.
After installing a plugin, go to Settings®Panel → Items and click the + button to see the "Add New Items" window shown in Figure 6.4f.