if you are admin and you can't change it, just click on the folder and get info (apple-i) and go to the ownership and permissions menu. From there, just click on the lock, type in your password, and select your user name. Then select read and write. Then select apply to all enclosed items.
if you are not admin, instead of using the main library, just use the library in your user folder.
hope that helps..
irmongoose