This is my first encounter with something weird...

I found an older book. When I put it in the system it says Enter all info...title, dimensions, publisher, etc. This exact book is in the system already but with a different ISBN number. So what do I do? Do I have to fill in all that info to be able to list it? Wonder if mine never gets chosen because it's different?

I don't know whether you have to list all of that information to post it to your shelf, but I will say that in the past when I've had to list a book under no-ISBN because it was older or had a different binding or the like, I've still gotten requests for it.

Does your book have any ISBN number at all?  If it doesn't, you will need to list it through the no-ISBN section.

yes it does!

Your book is simply an ISBN that has not been entered into the database before. Yes, it will still get chosen, and will not be a problem. And, yes, you need to enter all the information for your book (grab your ruler, dimensions are required).

Do open another browser to one of the other editions to grab the identical info by copy/paste to save you a bit of typing :)