Which version?

[matsumotojones]

Do I have to be playing on a particular version? 1.5.1?

I must be the noobest of the noobs cos I keep getting the Unkown host error and I've checked all the help threads and watched the tutorial.

 

[ItsTheSM1]

You have to use the newest version of Minecraft.

Try this: http://www.minecraftsurvivalgames.com/threads/cant-resolve-hostname-fix.17539/

 

[ShaunDepro]

The servers are compatible with 1.5.1 and I believe 1.5.

 

[K259864]

You have the right version, make sure you typed in the right IP, us1-50.mcsg.in

 

[HalfSquirrel]

The servers are compatible with 1.5.1 and I believe 1.5.

This is correct
It normally works as long as you are updated to the most recent non-bug fix update. Since 1.5.1 was bug fixes, the servers are compatible with both 1.5 and 1.5.1.

 

[AwesomnessFace]

Just log on again or restart your minecraft if you get the problem repeatedly.

 

[ejegun]

This is a problem with your DNS, please tell us if it got fixed after trying whatItsTheSM1 suggested. Because if yes, this can be locked, if no, this is my suggestion:

If you have hamachi, deinstall it.
Try logging in from a different computer and from a different network.
Try chaning your DNS server settings, there's tutorials on the internet for that.
I don't really know too much about this stuff, but you're not the only one having this problem, there is loads of people who have the same problem.