You have to log into your server using SSH and then go to that directory. Maybe you can use SFTP to get to it (if you can log into your server as root).
What you need to do is get a program that supports SSH.
If you're using *nix just go to a terminal prompt and type SSH [email protected], you should get a prompt to enter the password for root.
If you use windows get hold of a program called "putty" (do a google search), install it and set up your connection (remember that SSH is on port 22). You'll get a terminal window that will ask for a username and password.
If you want to use SFTP from a windows machine try "WSFTP" and specify SSH/SFTP in the connection type.
Reseller SKINS are kept in a different location and will not influence the serverwide skins.