You can retrieve MALLET here, although in the future you should probably get it directly from David Mimno’s github repository. MALLET can be used for more than just topic modeling, so do explore the documentation at the UMass page (the first link). Our original Programming Historian tutorial on getting MALLET up and running is at http://programminghistorian.org/lessons/topic-modeling-and-mallet.
Read about Mimno’s plans for the future of MALLET on his blog. Incidentally, Mimno has another implementation of topic modelling (LDA) in javascript that you can download and explore here.