If you Activate any Topup, you need to Send Just like Following Step
Add Topupcode and Send it to 54325 from your registererd phone no.
Exanple If you want to activate &prive HD
You need to send sms
ADD TOPUP7729 Send it to 54325.
Also By sending Topupcode You can Remove channels also.
You need to Type REM Topupcode send to 54325
For topup code details go to this thread created by @iphifi bro List of Airtel Digital TV topups available in myAirtel app
Bro First you need to Confirm the Regional Variant of your South New Mega HD your'e in
If your'e on Telugu (AP) New South Mega Gemini HD can be added as Free Sun HD Topup, incase of Tamil (TN) all 3 Sun HD Tamil Channels can be added as Free Topup via Customer Care.
If you aren't on AP/TN New South Mega you should be in any of the other two South New Mega namely Kerala/Karnataka New South Mega HD in that case you have to add as A-la-carte at Rs.40 per Channel or opt for Combo Topups everything about various Topups can be known in a bit detail from India's leading provider of Prepaid & Postpaid Mobile Services in India - Airtel.in
Read this Disclaimer from this link that has details about Free Infinity Sun pack and verify whether it's available for you,