Before you can create a flow in the Mobile Builder, you have to register your mobile app in Appcues Studio and create an App ID:

Log into Appcues Studio → Settings → Apps & Installation → Add App → Enter your App's name and select the type of app (e.g. Android, iOS or cross platform framework). → Click Save.  A unique App ID is generated and displayed.

You'll need to provide your Appcues App ID, in addition to your Appcues Account ID (Appcues Settings → Account ID) to your mobile developer so they can initialize the Appcues Mobile SDK within your mobile app.

Once you provide these two values to your mobile developer, they can use them to initialize the Appcues mobile SDK as per the documentation. Your mobile developer can then identify, track and send activity data to Appcues so you can view Event analytics and target mobile Flows as needed.