Say you have a radio button with the values "yes" and "no". You want every new record to default to "no" when the record is first created.
Open Manage | Databaes | Fields, find the radio button formatted field and double click it to open file options. On the auto-enter tab, enter "no", but without the quotes into the Data box. Now every new record will auto enter "no" into the field and the radio button formatting will show that as a selected radio button.
For manual finds where you directly enter your search criteria into the fields, you can't search for empty radio button fields. You'd need to add a copy of this field to your layout formatted as an edit box for entering the = as your find operator. A calculation field that copies the value of the field could also be searched in this manner.
A scripted find, on the other hand, can specify the lone = operator directly in your radio button field without the need for modifying the layout.
Here are some examples of scripted finds: Scripted Find Examples