That was quick!
mapAccumL was added to Data.Traversable in package base-4. GHC 6.8.2 uses base-3...
I think I will be forced to upgrade my GHC by hand... I just can't stand 6.8.2 anymore...
Just a heads up:
I did a cabal install Tensor on my Ubuntu box, and got the following message:
src/Data/Tensor.hs:333:18: Not in scope: `mapAccumL'
I will investigate a little more and let you know.