Please cite the mpt package by the following reference:

Wickelmaier, F. (2011). Multinomial processing tree models in R. Presented at the R User Conference 2011, August 16-18, Coventry, UK.

Corresponding BibTeX entry:

  @Misc{,
    title = {Multinomial processing tree models in {R}},
    author = {Florian Wickelmaier},
    year = {2011},
    note = {Presented at the R User Conference 2011, August 16-18,
      Coventry, UK},
  }