You have Lr Mobile from the app store already on your i?????
And you have the latest Lr 5.7.1 on your desktop??
Then click the Identity Plate to access the Sync on/off button.
I have purchased light room for my desktop computer.
Have you purchased a perpetual license or subscribed to a Cloud plan?
To access Lightroom Mobile you must subscribe to a Cloud plan.
"Does Lightroom 5 include access to Lightroom mobile?
Lightroom mobile is included as part of the following Creative Cloud membership plans:
- Creative Cloud Photography plan
- Photoshop Photography Program
- Creative Cloud complete plan
- Creative Cloud Student and Teacher Edition
- Creative Cloud for teams complete plan
If you have access to Lightroom 5 via your Creative Cloud membership, then you have access to Lightroom mobile.
Since Lightroom mobile is a service, it is available only via membership plans. Lightroom 5 perpetual license does not include access to Lightroom mobile"