Allocating range <String.Index> for NSRange

This is where I need to give it to NSRange

for user in tweet.userMentions
{          
    let userName = user.keyword
    if let startIndex = tweetTextLabel.text?.rangeOfString("@")?.startIndex
    {
        let range = startIndex...userName.endIndex
        var atrString = NSMutableAttributedString(string:tweetTextLabel.text!)
        atrString.addAttribute(NSForegroundColorAttributeName, value: UIColor.redColor(), range: range as? NSRange)
        tweetTextLabel.attributedText = atrString
    }
}

      

        
        
        
      

    

What can I do? Maybe this is another feature that for quick I am using swift 1.1, iOS 8.1 SDK

Update Still not highlighting text

 for user in tweet.userMentions{

                let text = tweetTextLabel.text!
                let nsText = text as NSString
                let textRange = NSMakeRange(0, nsText.length)
                let attributedString = NSMutableAttributedString(string: nsText)

                nsText.enumerateSubstringsInRange(textRange, options: NSStringEnumerationOptions.ByWords, { (substring, substringRange, enclosingRange, stop) -> () in

                    if (substring == user.keyword) {
                        attributedString.addAttribute(NSForegroundColorAttributeName, value: UIColor.blueColor(), range: substringRange)
                        println(substring)
                    }
                })
                tweetTextLabel.attributedText = attributedString
                println(attributedString)