Filemaker Go is compatible with the 3GS with iOS 5.1. http://help.filemaker.com/app/answers/detail/a_id/9333/~/system-requirements-for-filemaker-go
You must have Filemaker Pro or Filemaker Server running. The database must be open in Filemaker with sharing turned on.
See this link Cannot remotely connect to FMP 12 from iPad or iPhone
Thanks a lot S. The links you gave got me going after a thorough reading; turns out I hadn't correctly shared the Filemaker file. A very basic error.