Subdomains created in DirectAdmin are subdirectories under the public_html directory.
For example:
If your username is username and your domain is example.com, and you create a subdomain sub.example.com, then the full path to your subdomain is:
/home/username/domains/example.com/public_html/sub/
Your ftp path is:
domains/example.com/public_html/sub/
or if you only have one domain, simply:
public_html/sub/
I hope this helps.
Jeff