Mission Info modifies the mission tracker on the right side of the screen. When you hover over a mission's icon or text, you'll get a tooltip with the flavor text of that mission's current tier. Additionally, clicking on the currently active mission's icon will show any associated image, video, or audio clip. The Mission Journal can be opened by selecting the active mission in the mission bar (the part that auto-hides).
New official developer site: http://games.jupath.me/tsw/mission-info
Source code is available via git from curseforge.
If possible, please try to use the official forum for discussion in order to consolidate potentially useful information in one place.