For connecting TallyDekho mobile app with desktop app, you have to complete Account pairing as follows:

1. Open mobile application

2. Click on Setting

3. Choose Account Pairing

4. Select the option 'Scan QR code' or 'Upload QR code'.
If mail id on mobile and desktop app is same you will get your data 

If mail id is different,it will ask you to verify, for that a verification mail will be sent to the mail id on desktop app.
After verifying, you will get data on mobile.