You need to download and install PHP-Nuke first then.
You can get it at
PHP NUke
Install that first and check it to make sure it is running. BEFORE you try to port it.
Then go to the link I gave you before for bbtonuke and download the port.
You don't have to get rid of your current forum, you can import the data after you have ported nuke with the download on bbtonuke. The link I gave you earlier takes you to the correct place at bbtonuke to download the file.
Read ALL the readme files before installing.