Assuming you are using some kind of linux you need to use export

export TRINITY_BASE_DIR=/location/of/installation/directory

is it is searching for it in the cood here

if os.environ.has_key('TRINITY_BASE_DIR'):
    TRINITY_BASE_DIR = os.environ['TRINITY_BASE_DIR'];
else:
    sys.stderr.write("You must set the environmental variable TRINITY_BASE_DIR to the base installation directory of Trinity before running this");
sys.exit()

after exporting it you can see all the exported variable using this command

export -p