The Commons BeanUtils used by Struts to populate ActionForm beans will not populate an ArrayList automatically. Unless you want to mess around with the inner workings of Struts and the Commons BeanUtils, you really can't pass an ArrayList directly to an ActionForm.
The BeanUtils will populate a String array, however. I'd recommend you change your ActionForm property to type String. If your model classes really need an Arraylist, you can always convert the String array to an ArrayList yourself before passing it on to the model.
Here's an example:
In the JSP:
In the ActionForm:
When the form is submitted, myProperty will contain ["foo", "bar", "baz"].