Try designing a quick table, put in an autonumber PK, a text field, and use the following for your date field:
Field Name: DateTime
Data Type: Date/Time
Default Value: =Now()
Save and open the table. The Date/Time should automatically populate the DateTime field.
If you are opening the table in a form, and the DateTime field is not on the form, try putting it on the form and setting visible to No or False for that field. Make sure it is high in the tab order for good measure.
Does this help?