You would write code on the change event of the RadioButtonList (look in the hierarchy) where the radio buttons are grouped. The group object will hold the value that is currently set for the RadioButtonList. You can test to see if the group is set to 5 then you can make your field visible. Make sure that each RadioButton has a unique value when it is on. Something like this:
if (this.rawValue == 5) {
Fieldname.presence = "visible"
} else {
Fieldname.presence = "invisible"
}
Paul