The newest MineCraft release is 1.2.5, 1.2.3 is out of date. I would recommend using 1.2.5, which can be easily updated from minecraft, after logging in, since most servers have updated anyway.
Unless, of course, you use a cracked version of Minecraft. in that case, buy the game
After you update, use the "add Server" option. Enter anything you want for the Server Name - if the server you want to access is named BowlingBallCraft, for example, i would suggest entering it as "BowlingBallCraft", but anything works. Then, enter the IP address that the server host gives you in the "Server Address" option, click "Done", and join the new server! : )
