I'm not entirely sure if you're using the Mac OS X Server or the Mac OS X client software version here, so the following covers both. Use of System Preferences implies client, though this is a server forum.
With Mac OS X Server 10.6, use the Server Admin tool to open up the ports you need. Launch Server Admin > select the target server > select Firewall > select Settings > select Services, open the ports, Save, etc.
With Mac OS X client 10.5.1 and later and with 10.6 and later, the firewall mechanisms and settings are application-based and not port-based, and you select the application to allow, and it can then use whatever ports it needs. System Preferences > Sharing > Advanced... and then the + to add the application. (Details: [ht1810|http://support.apple.com/kb/ht1810])
If you're running this box as a network gateway, then things can get a little more complex; I'd tend to use an external firewall if that's an option.
If you're running this as the client for the disk services (as I might guess) and if you're accessing a remote web disk on a remote system via ports 2077 (SSL) and 2078, then I wouldn't usually expect to need to open these ports on the client; it's usually the remote server system and not the client that needs to open ports. (Are you getting any diagnostic messages here? Are you looking to serve disks out to other hosts via this protocol? If so, and if you're on Mac OS X Client here, then try adding the application that serves the disks to the list of applications known to the firewall.)
As an alternative, Apple servers and servers from other providers can run WebDAV disk services via port 80, which means that most any server that allows http web connections can also allow WebDAV connections. (WebDAV is a built-in feature of this Apache web server, so most any Apple Mac OS X box can serve disk space using http port 80 and WebDAV.)
The lower level approach to opening the ports is the ipfw tool, and the command line.
