You can use this library to convert NSString to NSDictionary. Import "SBJson.h" in your .m file and use following code
NSDictionary *dictionary = [u JSONValue];
NSString *userId = [dictionary valueForKey:@"userid"];
Edit
Alternatively you can use NSJSONSerialization
to convert NSString
to NSDictionary
NSError *e = nil;
NSDictionary *dictionary = [NSJSONSerialization JSONObjectWithData:u options:NSJSONReadingMutableContainers error:&e];