BuddyPress.org

copy the pot file from BP
rename it .po
edit it with a text editor
save it in the text editor
open it in poedit and save it
upload the .mo file per instructions here:
https://codex.buddypress.org/how-to-guides/customizing-labels-messages-and-urls/
create a file titled bp-custom.php and add the code as instructed in the above link
upload the bp-custom.php file to your WP plugins folder
done

PS
you should never have to open the .mo file