I can't post
If you are unable to see the + icon on a padlet, it means that posting is disabled. The owner of the padlet must change settings to allow posting for others. In this case, you may contact the author of the padlet to request posting ability.
If posting isn't possible for a padlet you've created yourself, check the current account you are currently logged in. Most of the time, it's caused by the system not recognizing the logged in user as its owner.