Ana,
Click the little arrow next on Add Books button > Create catalog.
Select CSV as output format, and on other tab select ISBN. For "insurance" against re-issued ISBNs, you may also wish to include Authors & Title. Before you import the result into Goodreads, open it and change the Authors column title to "Author" so it matches the fields GR can read.
Edited to add: I know you already found the instructions (I can read, I promise) but I put the full info there because of the re-issued ISBN problem that crops up a lot when I import into GR. Plus, others may find this thread and want the same info.
