Does anyone know how to set up functionality to allow multiple selections from a drop down list?
Drop downs are not used for multi selections. Use a checkbox set.
Hold down the Cmd (Mac) or Ctrl (win) key before making a selection from the list. Keep the key down while making your selections.
Drop down lists are not designed for selecting multiple values, but as this poster mentioned you can get it to work using the CMD / CRTL keys.
I'd add that simply selecting the values while holding down the appropriate modifier key may not actually select the values you choose - or more importantly, may not write the data to the field. In some cases it may be necessary to hit ENTER / RETURN to actually commit the selected values to your file. Your mileage may vary.
I'm going along with all of the others, dropdown lists are not a choice option if you want users to select multiple options. As Steve Romig points out they can be tricky to use, and I'll add, hard to display correctly, confusing for end users, and hard to adjust selections once they have been made.
I would highly recommend that another approach is taken.
One "other approach" is to list the values in a portal with a button set up to look and act like a check box. Selecting an item can either add a value to a list or add a related record to a table. Both such a portal or a check box group could be placed inside a popover if space on the layout is limited.
Retrieving data ...