You must In order to edit a photo it must be part of a collection. In order to be part of a collection it must be:
1. Part of a Lightroom desktop collection synced to Lightroom mobile.
or
2. Part of an iPad-originated collection (say from Camera Roll) that is syncing back to a Lightroom desktop
Lightroom mobile will require you to: Log in using a valid Adobe ID (30 day trial only for non CC IDs) and then Sync to a Catalog on Lightroom Desktop that is signed in and syncing. So, you can edit images which originate on your iPad, provided they are also syncing back to a desktop.