The current release of SpeedTree Engine for Clarisse does not include an installer, so a few files must be copied manually.
Copy the engine shared library file (libspeedtree_engine_v8.2.0r2.so / speedtree_engine_v8.2.0r2.dll) to your desired path and adjust your system's binary execution path to include the library file's path.
Copy the plugin file (libspeedtree_clarisse_v8.2.0r2.so / speedtree_clarisse_v8.2.0r2.dll) to your desired path and invoke the Clarisse executable with the -module_path option. It takes multiple paths and must include both the Clarisse default module path and your custom path. For example, if your custom path is /home/user/my_clarisse_plugins/:
./clarisse -module_path "/usr/local/bin/Clarisse iFX 3.6 SP6/Clarisse/module" "/home/user/my_clarisse_plugins/"