I had the same problem as you... I just installed Photopost today to use with my phpBB2 forum (version 2.0.11). I am the admin, but I could not access the admin panel. I searched here and came across this fix, but it didn't work for me.
I went in and checked the DB with phpMyAmin and found that only the user groups have been imported to the pp_usergroups table. The pp_users table is empty. So what I ended up doing was creating a new usergroups on my forum and then I manually added that group to the pp_usergroups table. I then set that group to have access to the admin panel (with me as the only user in the group) and voila! I had access to the admin panel.
It works perfectly fine to log in using the username and password from the forum, but Photo Post just isn't recognizing the permissions, I guess. And when I logged in to the admin panel, there is no listing for "Users" - only "User groups" (I looked in the online instruction manual and saw that there is supposed to be a "Users" listing, too).
I had no problems installing the program, so I'm guessing everything went well. I just think it's strange that the phpbb_user_groups table have been imported and not the phpbb_users table. Or is this how it's supposed to be? Will new registrations still be stored in the phpbb_users table?
Thanks for your help!