I use my site in other than english language, and if I edit my language messages.po file (EDIT with notepad or other text editor, code editor from cpanel) and then download it to my computer and open it with poedit (in ubuntu), then save it, the messages.mo file is generated in the same folder as the messages.po file is on my computer. Then I upload to my server the .mo file, overwriting the existing one. In 3.7.1 it works just fine.
Now, the msgid value from below, is an ID that gets replaced with the msgstr value if the language is not english. I don't know a reason why it shouldn't work with english also, on the same principle.
#: oc-includes/osclass/controller/register.php:68
msgid "The user has been created. An activation email has been sent"
msgstr ""
The msgid value is found in:
oc-includes/osclass/controller/register.php
osc_add_flash_ok_message( _m('The user has been created. An activation email has been sent'));
NOTE! If you change the msgid in register.php, and you use your site in other than english language also, you should modify the msgid in all the messages.po and .mo files of the other languages.