How many servers aren't showing up for you?
What are those servers, exactly?
There are three reasons why a server may be missed from the list.
Filters, like game type, full/empty servers, etc.
You can see filter controls above the server list.
If your mod is 1.4.7, but a server is already 1.4.8, server will be hidden.
No connection between you and a server.
Only server those were successfully pinged are showing up. If something blocking you from reaching a server, it won't be displayed. It includes (but not limited to) firewall on your computer (do Mac users have any firewalls?), ISPs banning IPs, IP-ban made by server owners.