Starting in Fuerte, OpenCV is not a ros package but a system dependancy. Double check which version of opencv you installed for ros and which version has snake. Also look around answers.ros.org, there might be a solution for you.
Similar question here.
Originally posted by Kevin with karma: 2962 on 2012-11-26
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Original comments
Comment by DavidLavy on 2012-11-28:
Ok, for what I understand ROS Fuerte has version 2.3.1, I was looking at version 2.4.0 ... But how can I implement that function, or how can I update my OpenCV version from Fuerte ... I have installed OpenCV 2.4.0 in Ubuntu 11.10 but my system uses OpenCV from Fuerte by default
Comment by tfoote on 2012-12-10:
You can't easily switch it. If you want to you need to check everything that builds on top of it out from source.