Leave it set to plain. Make all of your changes. Then change it to attributed.
Underline your text in TextEdit(cmd-U) and copy-paste it in Attribute Inspector > Label > Text > Attributed.
Select label -> Attribute editor -> Title = Attributed
Select the text of the label in the text view in the editor -> Right click -> Font -> check Underline
If the change is not visible -> resize the label
You can make text underline from storyboard like this. But when you change the text programatically it will override and underline will gone. So if you have to change the text on runtime. Do this.
For Swift 3
let text = myLabel.text
let textRange = NSMakeRange(0, (text?.characters.count)!)
let attributedText = NSMutableAttributedString(string: text!)
attributedText.addAttribute(NSUnderlineStyleAttributeName , value: NSUnderlineStyle.styleSingle.rawValue, range: textRange)
myLabel.attributedText = attributedText
For Swift 4
let text = myLabel.text
let textRange = NSRange(location: 0, length: (text?.count)!)
let attributedText = NSMutableAttributedString(string: text!)
attributedText.addAttribute(NSAttributedStringKey.underlineStyle, value: NSUnderlineStyle.styleSingle.rawValue, range: textRange)
myLabel.attributedText = attributedText
Here is the solution in storyboard: Open the Attribute Inspector (make sure label is selected), Change the dropdown value from 'Plain' to 'Attributed'. Now a a small text editor will be visible under the font of the label. Select the text of the label, right click and change the font to 'underline'.
I also have attached a screenshot and successfully underlined a text using storyboard in XCode 7.0.1
I personally prefer to customise my design(s) as much as possible. Sometimes, you will find that using the built-in underline tool is not easily customisable.
To do something like the following, I have posted an example in How to make an underlined text in UILabel?