The easiest way to install rtorrent (a command line bittorrent client) is to use the RPMForge Repository. You can install RPMForge using the following artcle How to install RPMForge Repository in RHEL/Centos.
Once you are done, you just need to run the following command to install rtorrent in your RHEL/CentOS (5/6):
yum install libtorrent rtorrent libsigc
libsigc++ is a dependency library for libtorrent. Yum should include it automatically in the installation. RPMForge has the compatible libsigc++ in their repository.