Please use the following for both the website and the RepeatMasker software package:

Smit, AFA, Hubley, R & Green, P. RepeatMasker Open-4.0.
2013-2015 <http://www.repeatmasker.org>.

or

Smit, AFA, Hubley, R & Green, P. RepeatMasker Open-3.0.
1996-2010 <http://www.repeatmasker.org>.