Sorry to answer you so late but I've given myself a few days of rest.
ABout your question, I must admit I've never done this kind of coding before. But as the matter seems interesting, I got a look around. And at java.sun.com I found the following answer to a question that is a lookalike of your own question :
GetParameterValues("name of checkbox") will return values of checked checkbox elements.
Example:
if you have following HTML-code:
<pre>
<input type="checkbox" name="foo" value="asd">
<input type="checkbox" name="foo" value="dsa">
</pre>
If only first one is checked GetParameterValues("foo") will return
String[1] with String "asd". If both are checked you'll get String[2] with Strings "asd" and "dsa" and so on..
ABout the activation of a textfield as soon as you have the checkbox marked, there is a
java solution :
you should program an ActivationListener who takes care of this. But I'm sure you can find javascript solutions as well and I wonder whether these won't be found easier.
One example of a javascript solution can be found at
http://forum.java.sun.com/thread.jsp?forum=33&thread=351903