Acido - Judging by the error, it looks like vBulletin may have added a "filesize" column to one of the other tables that is joined in that query. Try opening your adv_index.php file and looking for "filesize," (without the quotes) and change that to "attachment.filesize," (again, wthout the quotes). I haven't even looked at RC3 yet, but hopefully that will be the only issue. If you have any more problems though let me know.
jaliam - You sould use the instructions and make the changes to functions_calendar.php and the template modifications to fix that problem.
Symbian.info - The second bit of code should be correct. I think the "under" part may have been meant to be part of the instructions (Add under).