I connecting to my ftp server with Finder, entering login and password.
After establishing connection, i'm in read-only mode.
When i connecting with some ftp clients, like Cyberduck, i can write.
How it can be done, using Finder features only ?
If you don't mind additional software to be able to use FTP in Finder, there are solutions for that too:
Another possibility would be a free FTP implementation for MacFUSE.