Software

The R-package stima (simultaneous threshold interaction modelling algorithm) is available here.
When you use stima, please refer to:
Dusseldorp, E., Conversano, C., & Van Os, B.J. (2010). Combining an additive and tree-based regression model simultaneously: STIMA. Journal of Computational and Graphical and Statistics, 19 (3), 514-530.

The R-package quint (qualitative interaction trees) is available here.
When you use quint, please refer to:
Dusseldorp, E., & Van Mechelen, I. (2014). Qualitative interaction trees: a tool to identify qualitative treatment-subgroup interactions. Statistics in medicine, 33(2), 219-237.

Both packages are suitable for R version 3.0.2 and higher. More information about the R Software environment can be found here