i cant see any option for sftp (SSH file transport protocol) so i cant login to my server using ftp disk... (i just bought it because it seems right for my needs)

FTP Disk only support the FTP protocol. The regular one known as FTP and the secure one known as FTPS. SFTP has nothing to do with the FTP protocol, it is actually (as you seem to know) a SSH file transfer protocol. FTP Disk doesn't support that protocol. I don't if it will be added in the future.