I assume you're talking about xcf.trigramreactor.net. You have three choices:
1) enter a search term in the top left corner
2) browse certain limited categories in "Categories" tab
3) click on the rather heavyweight Gun Profiler.
99 % of my usage entering a search term, for example, when typing "Vampi" you get various choices related to Vampires (the monsters and the Vampire Castle mission). For example, by searching for and opening a mission you can see which enemies are going to be deployed there, what kind of properties they have (e.g. sniper/spotter characteristics, weaknesses) and what kind of loot there might be. You can also use it as a tech tree viewer, though the inbuilt one is better for that.