Xournal isn't in diablo extras yet... You'll need to add the chinook extras repository in order for it to show up as installable. Although, you might want to disable it after you install xournal. It could cause some library problems.
http://repository.maemo.org/extras
chinook
free non-free
(I don't remember what the field names are, and I'm too lazy to get my tablet and look :P)