But why are passwords not working for anyone?
Different versions of Snitz encode passwords in different ways. This is why my converter doesn't take over the passwords. So your users will need to use the PHPBB password recovery option.
However later versions of Snitz use md5 hashes to store the passwords, which is the same as phpbb IIRC. So if you wanted you could add the bit of code to copy the passwords from Snitz to phpbb (very easy, just add 1 line of code to get the password from the Snitz database where the code extracts the username etc, then add the var to the insert statement that puts it into the phpbb database - it's all well commented).
And can i change an admin username from the snitz board that is now on the phpbb board back to admin status?
The converter doesn't destroy the existing phpbb users. So when you installed phpbb you will have created the first user in the db, which is your phpbb admin user. Just log in as this user then make other users moderators as necessary.
Remember that my script merges userbases - this is described in detail in the docs.