The path is not valid in hg executable

I used the Mercurial software for my project. But I am having problem with the hg executable.
I am certain that I installed the mercurial correctly on my Mac OS X Snow Leopard PC.
The path for the hg executable is in “/usr/local/bin/hg” according to the error message, but the path seems to be not valid.
How can I correct this issue?

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"/usr/local/bin/hg" doesn't appear to be a valid hg executable
OK
