How can I activate the developer mode?

You can activate the developer mode in the About Box by clicking About from the menu under your username in the top right corner of Odoo:

Click Activate the developer mode

Alternatively, you can insert the character string ?debug in the URL after your domain name and before the number sign "#".
Eg: http://mycompany.my.odoo.com/?debug#id=27&view_type=form&model=sale.order&menu_id=110&action=89

The developer mode will display a menu at the bottom of each view giving you access to advanced administrative tools related to view edition, workflow edition, translation, javascript tests, debug information, record log, default values, user filters,...