I don't know if it is possible to get a drop down list to allow for multiple selections. I guess a script could do something. I tried with the normal way here in this file. The way to count a check box set is to use a calculation and set it equal to ValueCount(Multiple Field). I tested this by making two layouts, one that uses a check box and one is a drop down. The one that is the drop down seems to highlight the selections that were choosen by the check box set. However when I try to click or add more than one it clears all previous, but notice if I go to the layout that has the abillity to use the check box set to select, when I come back to the drop down version they are selected. Interesting! Also the drop down layout clears the current number in the counted field and sets it to one every time.
Test files: http://pctechtv.com/show/testCountVL.zip
As I further test it seem like pressing return after the selection allows for them to stay selected. This populates the field with a multi-key. Multi-key means multiple value. The field can then be counted with the ValueCount(Field). I am on Windows I have had where I test on my Mac ad it is totally different. I mean because I don't understand why I have to press return to get them to stay selected.
Thanks Chris, this actually works on my Mac, AWESOME. Much appreciated.