All depends on your situation. userfix is a script to realign userid's to the proper ones based off a username when you switch an integration which is really not a supported thing since we can not guarantee success or all users exist in some other forum. One would hope your forum import script imports all users without messing up their usernames.
If you have successfully installed photopost with xenforo things will work. I registered a username on your site and it authenticates fine but your not logged in because your cookies are all messed up.
On step 4 of the install you specify your forum cookie prefix and that should be xf_ you left it empty and thats your issue.
Your cookies in photopost should be set the way the cookie FAQ states on here so your cookies are sitewide and work everywhere.