Have you defined the 'radio button field' as a script button? If so, it will always trigger the script (that is, operate as a button) every time you try and click on it. You will not be able to access it as a normal data-entry field.
If Sorbsbuster's answer doesn't solve the issue, please describe what you've set up and how it is failing in more detail.
How to set that option.I just dragged a field control and changed the option button from Edit box to Radio button list.
If the field is a text field and formatted with radio buttons, it should be readily modifiable when you click a radio button.
You must have done something else in addition to that. You'll need to tell us what that something else is.
Here are some possibilities:
If you have specified a field of type calculation for your radio buttons, it will not be modifiable--you'll get an error message.
If you use a text field, but then select it and use button setup... to change the field into a button, it will no longer be directly modifiable.
If you select the field and specify in behavior on the inspector's data tab that prohibits entry into the field, it will no longer be modifiable.