Don't really understand what isn't working... what happens when you DO specify a directory name?
Are you sure that after wiping your disk and re-installing 10.6 (and you did install all updates, right?) - that you do not have Rosetta? If so, see this article on where to find it / how to install:
http://www.macworld.com/article/1167117/reinstalling_rosetta.html
You can also try downloading the AC 9 USA "converter package" here:
http://www.graphisoft.com/support/archicad/downloads/Fileconverter.html
which should be a ready-to-run installation, just copy into your Applications folder. It will not have the full US library, but you should be able to get it from your AC 9 installation disk or a backup. (For that matter, if you had a backup of your old disk, just restore the Applications > AC 9 folder from there.)
Looks like the library is also here:
http://www.graphisoft.com/support/archicad/downloads/library_updates/usaSEdownload.html
One of the forum moderators
AC 27 USA and earlier • macOS Ventura 13.6.9, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB