Before using any FreeSurfer command, including freeview (or tksurfer etc), FreeSurfer has to be set properly (in the terminal). The steps in bash are:

• export FREESURFER_HOME=/Applications/freesurfer
• source $FREESURFER_HOME/SetUpFreeSurfer.sh
• export SUBJECTS_DIR=/full/path/to/subject/dir
• cd to/the/functional/data/folder

Note: for FreeSurfer 7.1 or later, you may need to update bash if you are still using the default bash in Mac. You may follow the instruction here.