[R-SIG-Mac] help with ltm package

[Andrew Landers]

on my macbook, R won't run ltm package, it keeps telling me this:
> library(ltm)
Loading required package: MASS
Loading required package: msm
Error: package 'msm' could not be loaded
In addition: Warning message:
In library(pkg, character.only = TRUE, logical.return = TRUE, lib.loc = lib.loc) :
  there is no package called 'msm'

please help