To cite the nlmeU package in publications use:

Andrzej Galecki & Tomasz Burzykowski (2013) Linear Mixed-Effects Models Using R: A Step-by-Step Approach First Edition. Springer, New York. ISBN 978-1-4614-3899-1

Corresponding BibTeX entry:

  @Book{,
    title = {Linear Mixed-Effects Models Using R: A Step-by-Step
      Approach},
    author = {Andrzej Galecki and Tomasz Burzykowski},
    publisher = {Springer},
    edition = {First},
    address = {New York},
    year = {2013},
    note = {ISBN 978-1-4614-3899-1},
    url = {http://www-personal.umich.edu/~agalecki},
  }