Well I had been trying to do online check-in since Thursday without luck (always getting a message about filling out all the yellow boxes...but of course that helpful message doesn't tell you which yellow box is missing, or that it has a problem with).
But, this morning I cracked the code and figured out what it was complaining about...hopefully this will help some of the rest of you out:
-I finally realized that where the "fill out all the yellow boxes" error was specifically showing up was in the section with my address.
-There is an edit button you can click and you'll see that all of the yellow boxes are in fact filled out (at least they were for me), but I went ahead and retyped everything in in a different way so that it would replace the information already there (i.e. they had my address in all caps and a phone number with no dashes, I re-entered everything with just the first letters capitalized and a phone number with dashes) - then saved that.
-Once I did that, I was able to complete the online check-in!!
I hope this "workaround" can help some of the rest of you out!