Lets say we have a Lookup wizard field that appears as a set of radio-buttons. We want to display it on the search panel and make sure one of options selected by default. Here is the code that can help:
$("input[id^=radio_CategoryID][value=6]").attr('checked','checked');
In this code sample replace CategoryID with your field name (case-sensitive), replace 6 with the value you want to be selected. Note that if Link field and Display field in Lookup wizard setup are different you need to specify Link field value in your code.