That thread has more detail, but the immediate workarounds include:
Select "Google" as the search-database, and enter the title and author instead of the ISBN, as in: Tom Sawyer by Mark Twain
That may bring up a valid entry, though that doesn't always work either, but at the moment it's a better chance than using the ISBNs.
If that doesn't work, you'll need to use the "skip search" button and enter all the fields manually. I find I have to do this more often in the late evening (eastern-US time), while at other times of day the search function works pretty well.
Hope that helps! (And I hope the lookup bugs get fixed soon!)