What is learned from listening for inventory messages?
The inventory message only contains the transaction hashes or block hashes and an enum that specifies which type a given hash is. In the paper, when they talk about addresses, they mean IP addresses. What they are describing here is that the attacker takes note of the first couple of IP addresses (in this case, 10) that sent out inventory messages for a transaction hash.



Edit to answer your follow ups:


  
  Step: This runs parallel to Step 1-3. In this step I'm listening to all INVENTORY-Messages. So you said the INVENTORY-Message contains the transaction hash and an enum. How will be one  INVENTORY-Message generated? Will this message generated for every transaction and distributed by broadcast to every known node in the bitcoin network? Who creates this INVENTORY-Message?
  


The inventory message is just a message sent to a nodes peers. It is not forwarded to other nodes as it is not a message like transaction and block messages.

If you (as a node) receive an inventory message from me (as a node), that means that I am announcing to you that I have a transaction or a block which you might be interested in. If you want that transaction or block, you will respond with a a getdata message and I will respond to that with the transaction or block itself.


  Did I correctly understand the first three steps?


Yes, I think you did.