If this is text in a custom dialog, then it won't work. If this is text returned by a calculation field in your layout, it should work.
Text formatting, value lists in input fields, data validation while the dialog is still up are all not options we can use with Show Custom Dialog. That's why many of us go the extra mile and use New Window with some careful scripting to set up our own "Pseudo dialogs". These may not look as pretty as we'd like, but they allow for a lot nicer user interface when it comes to data entry.