It may be a glitch with your server-listing - it's been known to happen off and on.
As Frank mentioned, Ventrilo would be a good place to start and ask around if anyone else is having issues connecting, or finding a possible reason behind why it isn't showing up.
A few other things you could try (In no specific order):
Check for Steam Updates
Restart Steam
Check for OS (Operating System, such as Windows) Updates
Restart Computer
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-- Another possibility is simply just waiting anywhere between 10 minutes to an hour and if it is indeed a glitch, it should show up then.