To cite msm in publications use:

Christopher H. Jackson (2011). “Multi-State Models for Panel Data: The msm Package for R.” Journal of Statistical Software, 38(8), 1–29. doi:10.18637/jss.v038.i08.

Corresponding BibTeX entry:

  @Article{,
    title = {Multi-State Models for Panel Data: The {msm} Package for
      {R}},
    author = {{Christopher H. Jackson}},
    journal = {Journal of Statistical Software},
    year = {2011},
    volume = {38},
    number = {8},
    pages = {1--29},
    doi = {10.18637/jss.v038.i08},
  }