Easiest way is to use the Dialog editor (I presume you used it to create your dialog?).
With the dialog displayed in the editor, right-click on the control you want to change (or for the whole dialog, anywhere on the background) and then click "Attributes" in the pop-up menu, then -> "Text Appearance" tab -> "Font and Color Selector" button and select the Fixed font you want... something like "Fixedsys" or "Terminal" and the size, color etc.
It will put the correct stuff in the Dialog command-lines for you, otherwise you would have to do the tricky numbers by yourself ... something like "Fixedsys|9728|40|49"
Hope that makes sense!