There is not a "default font" in WordPro. What you can do is modify the Default Text Style in the default smartmaster. You have not said what version you are using but to change this in Wordpro 9.5 you do the following :
1. Open up WordPro and create a new document (MUST DO THIS).
2. Then open the smartmaster file called 'default.mvp(default location for this is c:\Lotus\smasters\wordpro\default.mvp')
3. When open type a few characters on the page and highlight them.
4. Change to the font that you require as your default.
5. Whilst still highlighted, text properties and the style sheet tab (right most tab).
6. Make sure Default text is highlighted in the list of styles and then click redefine style.
7. Delete the characters from the page from that you added earlier and then Save the changes.
This will change the default text and when you create a plain document it will be in your new chosen font.
For even more utility, I suggest you go to the Help files and look up Styles. Start with the Overview: Styles, then Overview: Paragraph styles. Then on from there if you wish.