Hi Emily,
You need to open your form in ArcPad Studio, then, open up that date and time control properties. When you switch to the Values Tab, you'll see a default value field. So, just enter the Now() funtion in that field. Each time user opens the form, that field will be populated with a current date and time.
If you want to populate a current date only, use the Date() function. Also, Time() function will return a current time. Attached is a screen capture of the control's properties.
