So, is your old board located inside a subdirectory of the location where you installed phpBB3? My guess is, it is not
Please not the explanation below the "Board path" label:
"This is the relative[/i] path on disk to your former board from the root of this phpBB3 installation".
This means that if your phpBB3 installation is in /phpBB3 and your phpBB2 installation is in /board, you would enter as the board path"../board".
The double dots are to move you up one level in the directory hierarchy.