It's kind of sad that I finished this up today mostly out of petulence rather than wanting to help.
Localisations are probably off because I obviously haven't travelled every path in the First Gen, but all pertinent information and CG locations and, yes, that damn hotspring event should be in there. (For those of you baying and howling for it, you do realise it'll tank your love rates, right? And that you can get the CG from a title later anyway? Just checking...) Feel free to go join up for the wiki and be helpful and edit stuff up.
If you mean the one in the middle of part 4, then it's a choice as part of the plot and you can't miss it if you're going that route. From your mention of Mudus Gate this probably means you either did it already or you took a different path through part 4.
If you mean the one at the end, you need Elaine at max rate.
No worries - in the end, the main work was just the grunt work of copying everything up and wrestling with Wikia's terrible, terrible editor. Of course, the fun comes when I tackle Generation 2 and the mound of Japanese on those events they're not sure how to get...
On first generation, for the hotspring event, Elaine needs to be at like or higher love rate. It doesn't work with normal love rate (I also had Fyuria at like and Luana at love so maybe all three girls need to be at like or higher to get that event).