My installation of R 2.8.1-1 is in progress as I write this. The original instructions for installing R on FreeBSD are at the R Wiki, but before you dash there, you may want to know that I had to make some changes. They are listed below:

First, the good news: on my FreeBSD 7.0 installed in February of 2008, I did not need to make any changes to the PATH, nor did I have to set the symbolic links mentioned in the instructions right after the paragraph that begins with "A word of warning [...]". You may have to, but check first:
 # whereis make # whereis sed 
If your system is anything like mine, make and sed are in /usr/bin. Next, you need to check and see if the PATH includes /usr/bin, like so:
 # echo $PATH  If the answer is yes, you're OK. If the answer is no, before you make changes to the PATH, check which shell you're using, like so:  # echo$SHELL 
The instructions use the "export PATH" command, which is for the bash shell. The csh equivalent of that command is "setenv PATH". Finally, the symlinks mentioned in the instructions map your gsed to sed, but I have no such thing. I have psed instead. You can check what kind of stream editor you have like so:
 # ls /usr/local/bin | grep sed 
 kern.maxdsiz="734003200" 
 # cap_mkdb /etc/login.conf 
 # shutdown -r now