The error message on the forums is because the configuration file hasn't been written to by the installer; it's looking for a variable (the path to the folder) which doesn't exist, so it's trying to load it anyway.
As it looks like the installer is stalling at the database steps, please ensure that your username, password, and database information are all correct; UBB.threads will not create a database for you, it will simply insert the forum information into the database to get you started.
As for the installer, could you try re-uploading the UBB.threads archive that you downloaded from UBBCentral; I just ran an upgrade with the files last night, and Isaac ran a couple of fresh installs prior to release, so it should work. If it cannot continue again, please check your hosting with the
System Requirements Test.
