I assume you are in the command tab for Photo Montage, right?
click the Edit List Button in the Transition FX section. open the Sony Dissolve part of the list, select Additive Dissolve and Click Add>> to add it to the list on the right.
When you click Okay, it should be the only plug in showing and will be applied when you click apply at top.
Regards,
Mary Waitrovich
UW-Madison