If the string you are looking for cannot be found in either the php pages/ code or the DB Dump then try this:
This usually means the string is being stored at the DB as an object and/or somehow encrypted.
This in turns means the string should be accesible via the GUI.
Try the following:
1.- login as admin
2.- on the cog menu, go "Navigation"
3.- on the left menu go "Main menu"
4.- at the "home" item, click on the url address "/index.php?redirect=0"
5.- scroll down and find the html button
6.- On the cog menu for ht block where the button is in, select the "Configure resource..."
7.- Edit the button as required.