Hello, Im a noob with fourms and I keep getting this error?

Install phpBB2 (2/3)

The installation can not be completed:

- No valid directory name provided.

Click on the browser's Back button to fix reported errors.

I think im doing some thing wrong. Im trying to install it inside my public_html is that where it goes. I dont want it to be in my root directory I want it to be in my sub. Do you have to create your sub directory? If so please tell me how. Thanks!