Purchasing license number is required if you want to use the Expert Advisors on live accounts. You can download the demo versions first and test them on demo accounts. After purchase and after successful funds transaction confirmation, you'll receive a license number by email so make sure to provide a valid one. You can use the Expert Advisor with same license number on up to 10 accounts/computers. It is advisable also to disable ad-blocking temporary or add both www.paypal.com and www.algotradekit.com in the whitelist in ad-block's settings before purchase.
Payment is made via Paypal and before proceeding you'll have to accept and agree with Terms and Conditions.