To cite package ‘mvtnorm’ in publications use:

  Genz A, Bretz F (2009). _Computation of Multivariate Normal and t
  Probabilities_, series Lecture Notes in Statistics. Springer-Verlag,
  Heidelberg. ISBN 978-3-642-01688-2.

A BibTeX entry for LaTeX users is

  @Book{,
    title = {Computation of Multivariate Normal and t Probabilities},
    author = {Alan Genz and Frank Bretz},
    series = {Lecture Notes in Statistics},
    year = {2009},
    publisher = {Springer-Verlag},
    address = {Heidelberg},
    isbn = {978-3-642-01688-2},
  }