As the compiler is trying to tell you is that it's expecting a NSURL
object but instead you are giving it a NSString
object.
So you can try to convert your NSString into a NSURL by doing something like this:
NSString *urlString = afeed.url;
NSURL *url = [NSURL URLWithString:[urlString stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]];
Then you can do:
NSXMLParser *urlParser = [[NSXMLParser alloc] initWithContentsOfURL:url];