Do you have this working on any computer that uses Windows 7? You may need to upgrade to a newer version of FileMaker. If cost is an issue, you need upgrade only the machines with Windows 7 installed as both Filemaker 11 and 7 use the same file formats and thus can access the same hosted files over the network.
You can also download the 30 day free trial to one of these machines to test and see if it works for you before actually purchasing the new version.
If you have gotten FileMaker 7 to work with Windows 7, please chime in if you know what can be done to make this work.