How to play Minecraft 1.7.4?

I don't know what to do.
I open the minecraft launcher then click play. A green bar that looks like a loading bar comes across like it's loading something, however when it finishes it does nothing. I've tried this several times and I'm still stuck.
Any ideas?

Alex

Update your launcher. Only the latest Minecraft Launcher (version 1.3.9) has the proper protocols to download 1.7.4 data. You can download the newest launcher from minecraft.net

Kevin