
Activate Developer Mode in Odoo 9 or older
Click on the user in the right corner, then "About" and "Activate Developer Mode".
If you can't do this you should activate "Technical Settings" in the Settings/Users.
Odoo 10, Odoo 11 & Odoo 12
Since Odoo 10 the Developer Mode is activated from Settings.
Activation link is located just below the "Share the Love" box.

How to activate developer mode in Odoo
Multiple versions covered
Odoo