What are the guidelines in layer selection to type inside a shape?

Hi, I am using Adobe CS4 on an OS X 10.5.8., and I got a font issue.
If I want to type inside a crescent shape created by myself using the Circle tool and the Subtract pathfinder action,
I get the message: “You must click on a non-compound, non-masking path to create text inside a path”. Can someone tell me what I did wrong and what is the good method to do this?
Please provide me with procedures so that I may not encounter this anymore.
Thank you
