I am guessing here, but I believe that the error you encountered, as stated in the E-mail you recieved, is because of a small error when you started the forum install process.
In the control panel >goodies > one click installs you select PHPBB and then specify where to install it at. You first select the domain. And then you typed in the box for a folder. It is my guess that you typed "Forum/" or "/Forum". however it should simply be "Forum".
If this is the case, and you put in a "/" then you will need to follow the steps I gave in an earlier post again to install your forum.
Also, I'm not sure if the install script created your database allready or not.
If the database has been created, you will see it in the list of databases, and you can simply select it, instead of filling in the information to create a new database - that would be instead of step 3 above.
So, basically if you don't remember how you typed in "Forum" in the panel, just try the process over again. If you do remeber and you did type in "Forum" Not "/Forum" or "Forum/" I think you will need to contact support and find out what is up
- enless you want to learn how to install the software your self. Then you can download the zip from phpbb, extract, configure, upload, Chmod, configure, and then be in good working shape. But that's a bit more complicated