FMS comes with 1 concurrent license for Filemaker Go and Web Direct additional license will have to be purchased for additional concurrent users. http://www.filemaker.com/products/filemaker-server/docs/concurrent_connections_datasheet.pdf
There is nothing special you have to do to access a database from Go that is being host by FMS. Use open remote just as you would on a computer.
Filemaker Pro can have a total of 5 concurrent users, this is for Pro and/or Go users.
Thank you very much