Android - How to enable developer mode to install apps via ADB for this cheap Android Watch?

If its an Android Wear watch (and it seems so) there is no options on the watch directly. It depends on the phone it is paired with, and all the options are available on your phone.

I have a Moto360 and I also don't have any developer options on my watch. If you plan to develop for Android Wear, you need to install everything on your phone, which will push all needed resources to your watch.