The missing code is as follows:
NSError *error;
NSData *xmlData = [NSPropertyListSerialization dataWithPropertyList:borrowed
format:NSPropertyListXMLFormat_v1_0
options:0
error:&error];
if ( !error )
xmlString = [[NSString alloc] initWithData:xmlData encoding:NSUTF8StringEncoding];
There is no need for any special framework to be linked in.