Note: there is also an FM GO Forum (See tabs at top of this screen) specifically dedicated to FM GO questions.
To make multiple selections from the value list (not something I'd recommend for a drop down list in FM Pro, let alone FM GO), You'll need to design a different approach as there is no method for doing so from a drop down list in FM GO.
A field formatted with Check boxes would be the simplest to set up. It may not work in all cases as the field will have a fixed size--unlike a drop down list, but it does support making multiple selections.
Another option is to set up a portal that lists all the values for your drop down list with a button in the portal row to tap to select a value. With a bit of conditional formatting trickery or the new "hide object when" setting found in FileMaker 13, this button can be made to look and function like a check box, but since this is a portal, you can use a scroll bar to handle larger lists of values than might otherwise fit on your screen. (Especially if you iOS device is an iPhone 4s.)
Another trick for portals in iOS customized layouts is to position the portal so that the scroll bar is to the right of a tab panel or the layout boundary so that the scroll bar is not actually visible. This saves precious screen space, but in the iOS touch environment, you can still scroll the portal by "flicking" it up or down.
Note: A very similar interface can be set up with a list view as well where you put a selection button in the body of the layout.