Quote from shaizo
replace your minecraft.bin file with it and rename it to minecraft.bin. the URL to the minecraft.bin file is %appdata%/.minecraft/bin/
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That is incorrect.
The file should be named minecraft.jar. If you are using IE8 or 9, it'll download the file as a zip, forcing you to unhide the file extensions in Windows (Start, control panel, search bar for "folder options", Click show hidden files and folder link, and change uncheck the box "Hide Extensions for known file types"). Then rename it from .zip to .jar.
Then open a folder and type %appdata% in the address bar and hit enter. Navigate to .minecraft/bin/ , rename the minecraft.jar file already in there to minecraft.jar.125 (keeping it as a backup and marking it as ver 1.2.5). Then copy the snapshot jar into the bin folder. Once thats done, just start minecraft as normal.
If your using a 3rd party loader like Technic or MMC, make sure you understand how Instances work, and where they are stored, then put the snapshot in the correct instance folder.