That error is usually caused by Freesurfer not being loaded on the path. The typical way one does that with Freesurfer is something like this:
Bash:
export FREESURFER_HOME=/Applications/mri/freesurfer
source $FREESURFER_HOME/SetUpFreeSurfer.sh
tcsh:
setenv FREESURFER_HOME /usr/local/freesurfer
source $FREESURFER_HOME/SetUpFreeSurfer.csh