well well well... to be honest I tried a couple of times to install phpBB and still I see a few bugs in the redirections after logging in/out.
However I haven't found any bug in the install process and importing data from a "real" phpBB database.
What kind of version are you using? Are you using the "mambo component" one I hope... you should easily find it at this link:
phpBB Component.
You only have to extract from the .zip file the component and the module archives and upload it from mambo. Following the readme.html included is easy.
After installing you can decide if you need to import your data from another pre-existant phpBB or if you like to start from scratch. I tried both without problem and from the mambo admin panel you can export you phpBB user data to Mambo once for all. After that very first time, users who register with Mambo get registered to phpBB and viceversa.
It works great. I only found a few bugs but not related to the data migration operations.
I hope to have been of help,
Fred AKA MrDisaster
PS: extracting a .tar from a server sometimes doesn't work because of a server limit. Don't know If it's your case.