I'm attaching a sample for you to look at, perhaps that will help. You want to enter default value formulas using the "fx" button, so that they are added in the formula editor.
Right click the link to the file in the header of this post. Save the file locally. Right click the saved file, and select Design to open the file up in design mode in InfoPath. You'll get a warning that you can ignore. Preview the form to see it.
The template is for IP 2007 -- if you are using 2003, please let me know, and I'll save it in that format.