Signature Button Readonly until radio button is checked.

[#1]

I need to set a signature field as "readonly" until one of three radio buttons is selected.  I have tried the following script and cannot get it to work.  Any help will be GREATLY appreciated.  I have tried this on the "initialize" and the "click" event of the signature field and I still cannot get it to work.

[#2]

Looking at the object pallette, it doesn't appear that signature fields have an access property. Try using the presence property. Make it hidden by default then visible when the radio button is selected and vice versa.