You can't change plain text line spacing once it is already created. For both PText & MText, you can use Change, Text, Line Spacing or Ctrl+Right Click on them, Properties, and change the Factor.
'Factor' really should be changed to 'Line Spacing' in that dialog box to make it a bit more clear.
Thanks! - Josh Do. Or do not. There is no try. Josh's Digital Downloads is come back online soon. Stay tuned. (soon is a relative term)