Two things to also check.
Make sure if you recently upgraded vb
as well to 3.5 from vb2 that Photopost is indeed integrated with vb3 not vb2 by checking the settings table of Photopost and settings setting 41 to vb3
Make sure to edit your config-inc.php file and make sure the $vblicense variable is set to your proper vb
license ( this can be found at the top of the config.php file of the vbulletin install in their includes directory )