To enable the dev option, you need to use --dev=value from the command line. Possible values for this option are all, reload, pudb|wdb|ipdb|pdb, qweb, werkzeug, and xml. Take a look at the following example for more information:

$ odoo/odoo-bin -c ~/odoo-dev/my-instance.cfg --dev=all

If you want to enable only a few options, you can use comma-separated values, as follows:

$ odoo/odoo-bin -c ~/odoo-dev/my-instance.cfg --dev=reload,qweb