ROS C-Turtle can be run on FreeBSD, but the FreeBSD packages currently only support Diamondback. See diamondback/Installation/FreeBSD
ROS (the equivalent of ros-diamondback-ros on Ubuntu) is up and running on FreeBSD. A port (package source) has been added as devel/ros. Additional packages probably run too, given that they do not contain operating system specific code.
Add the variables to your environment after installing the port, as shown by the port. This enables running and building additional ROS software.
You might also want to read in the contents of e.g. $ROS_ROOT/tools/rosbash/rostcsh to get the ros* shell extensions:
Currently the stacks in the table below are available. Follow the instructions on the link to install them, the link also contains the current version of the port. Unless indicated otherwise, all packages in the stack have been ported without any "build" directory, and the stack is at its diamondback version.