With the release of the new 1.6 patch for Minecraft, Mojang has released an all new launcher for use with the game. Needless to say, you'll need to do a bit of tinkering if you want to use your old 1.5.2 mods in the new version of the game.
Keeping that in mind, the Minecraft community has quickly devised an easy guide to convert existing mods to use in the new 1.6 launcher. Here's what you'll have to do to get them to work:
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Copy the 1.5.2 folder (call it "1.5.2-Mod")
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Rename the jar from "1.5.2.jar" to "1.5.2-Mod.jar"
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Rename the natives folder from "1.5.2-natives" to "1.5.2-Mod-natives"
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Rename the json from "1.5.2.json" to "1.5.2-Mod.json"
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Open the .json file and edit the ID line so it has the value "1.5.2-Mod"
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Modify the .jar as you would normally
Unfortunately, it doesn't work for every mod, so you'll have to keep that in mind when trying them out.