Using global actions I have been able to add custom buttons to record level layout, but I didn't get how to add a custom button to list view layouts in saleforce1. Thanks in advance.
Global Actions now have their own menu section under Setup | Create | Global Actions. Custom Actions however are still added to Salesforce 1's menu by going to the page layout for the object they're applied to. List views don't have the option to add custom actions as they don't apply to List Views, only to Object Records.
Once you're on the page layout the Action is related to, depending on your settings, you may or may not see the Global Action Layout. If you do, directly underneath it, you'll see the Salesforce 1 Action Layout. You can drag and drop any Object specific actions onto that menu. See the screen shot below from an Account Page that has no Global Actions.