WorkingOn is currently only available as a web application. There could be an app in the future, but right now we are focusing on integrating with products like Slack to improve workflow.
If this feature makes a big difference for you, please let us know. We really appreciate and pay a lot of attention to feedback.
For on-the-go usage, we recommend using the /on command from within Slack or posting by sending an email to firstname.lastname@example.org.