as far as i can see there isn't a way to put them in, you could put in marks such as f, ff, p and so on and then just write dim. (and the term for getting quiter in diminuendo)
There are buttons for f, ff, fff etc. separately. It won't be seen on the tab (except as grayer notes), just in the midi playback tho. The buttons should be in the lower right corner of the toolbar (I think I have default layout).