The new 1.7 update changes the TNT so that hitting it does not activate it anymore. You need to place a button / lever / redstone torch next to it to detonate it now.
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you need to be in multi-player or a server to make the TNT work.
The ID (code) for TNT in Minecraft is 46.
You can set TNT off in Minecraft Beta by clicking it, or by triggering it with redstone.
Settings for MC.
You Need to use an torch then right click the TNT