This function will return the topic pane. This function was created by Adobe for the Toolbar scripts.

function GetTopicPane() {//Get the topic pane
    var topicPane;
    if (top.frames[0].name == "ContentFrame")
        topicPane = top.frames[0].frames[1].frames[1];
    else
        topicPane = top.frames[1].frames[1];
    topicPane.focus();
    return topicPane;
}