I've installed Odoo version 10 module but I didn't see the activate developer mode under About section.
Developer mode has moved from the user screen to Settings
in Odoo 10+ version.
Click on link "Active the developer mode" or "Active the developer mode (with assets)"
Here is reference for Odoo 10 and will be same for Odoo 11 and 12.
In Odoo 13: