Just a note on this. I downloaded the “portservertsfamily.vss” file from here: http://ftp1.digi.com/support/documentation/portservertsfamily.vss but when I open that file in Visio it appears blank.
Just ran into this myself but found that when you download the file it comes down as a .vsd. Change its extension to .vss and then drop it in your my shapes folder. Works after that for me. Hope this helps!