You only need cmake on your BBB if you are compiling on it, I personally prefer to cross compile in a most powerful computer and after that send the binaries to the target, in this case BBB.
If you dont know how to do that perhaps a little investigation is the way to solve it.
For cmake on your BBB
tar xzf cmake-2.8.11.tar.gz
cd cmake-2.8.11
./configure
make
make install
For this you need developer packages on your BBB, which increment your footprint.
By the way, you should probably only post on one thread and no all threads that say OpenCV, just to keep conversations clean.
Regards
--Allan Granados