Hi,
First off, it would be better to set this at design time in Designer. See the Object > Field palette.
If you need to dynamically change the mark style, then there is an example here where you can set the mark style of the checkbox using script: http://assure.ly/kp60nk (see the third set at the bottom of the page).
The docReady event would be a good event to use for this.
this.ui.oneOfChild.mark = "cross";
Hope that helps,
Niall