Your solution "should not" work for my problem, as it was a matter of the server not beeing able to create new directories if it had a trailing slash. This is a known issuse with some hosting providers. The problem you had seemed to be about the persmission to upload files...
Did you extract/unzip the file before you tried installing it? I forgot to do that and got the same message as you. After extracting it and uploading the com_ file it uploaded fine.