Christine, that TWBX only has the CSV file in it. Could you post the KML and the CSV as 2 separate files? Then we can play with the joins.
Can you tell me what the error says? It might be that your join fields are different types and you just need to change the type on one of the fields you are trying to use (e.g., you cannot join a string and an integer).
And, as Michael mentioned in his reply, if you can attach your spatial file as well it is easier for us to look at potential solutions to the join problem.