Vc já tentou usar o método setSelectedIndex(int index) ?
setSelectedIndex
public void setSelectedIndex(int index)
Sets the selected index for this tabbedpane. The index must be a valid tab index or -1, which indicates that no tab should be selected (can also be used when there are no tabs in the tabbedpane). If a -1 value is specified when the tabbedpane contains one or more tabs, then the results will be implementation defined.