setValue:forKey: is expecting an object for its value argument and you are passing it a primitive (in this case, an integer). You can work around this by amending your method to read as follows:
-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
if([segue.identifier isEqualToString:@"detailSegue"]){
NSIndexPath *indexPath=[self.tableView indexPathForSelectedRow];
TRDetailViewController *vc=segue.destinationViewController;
vc.item=self.items[indexPath.row];
[vc setValue:self forKeyPath:@"delegate"];
[vc setValue:[NSNumber numberWithInt:indexPath.row] forKeyPath:@"itemIndex"];
}
}
And then access it like:
[[vc valueForKey:@"itemIndex"] intValue];