Yes, BookMooch is still alive and kicking; I've been listing some books and getting some requests recently. For a while I went the "hand-enter" route which is possible but exasperating. Even when I find a book with the ISBN in Amazon UK the information is usually incomplete compared with the Amazon US listing. Recently I discovered a system that works most of the time:
1) Look the book up in "Search BookMooch for a book" under the "Browse" tab.
2) If the book has ever been listed on BookMooch, it will appear.
3) Click on the book and you will get whatever information BookMooch has available. (I suspect most of these were drawn from Amazon US when it was still accessible, so the data are pretty good.)
4) On this page, if your book matches up with the description, you can click "Give My Copy" to have the book added to your inventory.
5) I guess if you have a paperback when the previously listed book is a hardcover, or something like that, you can note this in "Condition Notes" once it's in your inventory.
I still sometimes have to "hand-enter", but not nearly so often, and everything helps.