if ([WXApi isWXAppInstalled]) {
    NSDictionary *dic = responseObject[@"d"];
    PayReq *req = [[PayReq alloc] init];
    req.partnerId = dic[@"partnerid"];
    req.prepayId  = dic[@"prepayid"];
    req.nonceStr  = dic[@"noncestr"];
    req.timeStamp = [dic[@"timestamp"] intValue];
    req.package   = dic[@"package"];
    req.sign      = dic[@"sign"];

    [WXApi sendReq:req];
}

req.package 所需参数是：Sign=WXpay。

参考博客：http://blog.csdn.net/minggeqingchun/article/details/52774481