There are various external packages that are used for fitting. Here are the instructions to install some of them.
When using MultiNest please make sure to cite their works (arXiv:0809.3437, arXiv:0704.3704, arXiv:1306.2144) as well as the wrapper PyMultiNest ( Buchner et al. 2014, A&A)
To install the software please go to http://johannesbuchner.github.io/PyMultiNest/