First you must place the file into the DropBox folder on a PC or Mac.
Then download the DropBox app from the iTunes Store.
Open the DropBox app on your IOS device and locate the Filemaker File, I use a specific folder in the DropBox folder on my laptop named...."Filemaker 12"
When I tap on the F12 folder in the DropBox app I see a list of files. Tapping on one of those causes DropBox to download the file and then it says it is unable to view the file. Just tap on the Box with the swooping arrow in the bottom right corner of the display. This opens the file in the appropriate version of GO on your IOS device. It also moves the file into the Filemaker Go directory and you can then open it later using the Filemaker GO browser.