If you mean on your own Dreamwidth installation, you probably need at least to set userdomain => 0 in %CAP_DEF (in file ext/local/etc/config.pl, which you should have copied from etc/config.pl.example when setting up your site) and make sure it's not overridden in %CAP_PAID or %CAP_PREMIUM (same file). There may be other configuration items you need to change. I haven't looked at that closely in a long while.

Note, however, that Dreamwidth hasn't used subdirectories instead of subdomains for (IIRC) the whole 12 years and change it has existed, so there's no guarantee that the code handling that still works, or is fixable if it doesn't.