I'm trying to add 古蹟 to a list (http://beta.skritter.com/vocab/listsect?list=187169912&sect=9), but it is always corrected to 古跡.

古跡 is not used in Taiwan and I want to be able to add 古蹟.
I can figure out how to add this to my own list, but I want to have the correct work in the List because it is a published book.

I've switched it from 古跡 to 古蹟, so you should be able to add it now. If you see this kind of mistake (wrong traditional variant in Skritter for a multi-character word), you can send a correction to Evan, too. Just keep in mind that for some characters (like 着/著), we can't fully support all the traditional forms.

Do I need to clear the data in the iPhone app to have it show up there? (I haven't checked since there isn't an easy way to check words)

I think it will automatically update the traditional form within a couple days, but that's a bit complicated, so if it still shows you 古跡, then try clearing it.