I think it is fairly self explanatory.
You can't specify the value of an html:hidden component by nesting a
String value. They didn't allow for that.
You will just have to use the value attribute and a runtime expression. either <%= expr %> or If you have a JSP2.0 container then an EL ${expr}
<html:hidden property="param1" value="${param1}"/>
or
<html:hidden property="param1" value="<%= pageContext.findAttribute("param1") %>"/>
you might have to convert/cast the value to a String in the <%= expr %> to be a string.
Cheers,
evnafets