[Geoff Bacon]

You need to specify that you want this extra field to be displayed on the edit page.

Edit a song and tap "Configure Fields" on the top line.

I suspect that the above is what you want but, you could also specify that this field is shown on the library screen

In the full manual, the "Configuring Tabs" section says
"Both the number of tabs shown and their order can be configured in MobileSheets on the settings screen. To access the tab order dialog, load the Settings through the action bar overflow menu then tap on the “Tab Selection and Order” item under “Library Settings”."

Cheers
Geoff