From the command line use
(Note that these must be completed on separate lines.)
This will insert on the first line of the file.

sed '1i\
your text goes here' file_name > new_filename


This will append on the last line of the file.
sed '$a\
your text goes here' file_name > new_filename


Note also that with sed you have to direct the output and rename the file back. So 'mv new_filename file_name' when you are done with these commands.