OK, I believe you.
Something is moving now, we will see.
Now, when I opened a new wallet it automatically gave me a new receiving address. If I click "add new" it gives me a random one.
How can I add the old address in order to check whether I received the airdrop?
As I understand it, you need to make a backup of the old wallet first and then, after you installed the new wallet, locate the
wallet.dat file and replace it with your backup.