The data from the Millennium Simulation merger trees presented in Cole, Helly, Frenk and Parkinson 2008 MNRAS (383,546) (astro-ph/07081376) are available here.

The code implementing the merger tree algorithm described in Parkinson, Cole and Helly 2008 MNRAS (383,557)
(astro-ph/07081382) is available at the following links. Please read the README file and download file,  Code.tar.gz ,
which contains the code. 
Acknowledgement: Work that makes use of this code should reference
Parkinson, Cole and Helly 2008 MNRAS, 383, 557 and also acknowledge the "GALFORM Team" for making the
code available.

The code for the original GALFORM (Cole et al 2000) merger tree algorithm is still available at