Are you trying to retrieve comments from a SharePoint Task item?

Yes, the end users add comments specially if the are rejecting the item and I don't know how to later on retrieve them or show them in the SharePoint List....

You should be able to create a SmartObject out of the SharePoint tasks.  Site Actions > K2 Site Settings > Site Settings > K2 SmartObject Site Lists and Libraries.

I just created one out of the tasks...you can access all methods and data from that object.