What are the exact steps you are taking to access the file?
On the computer where the file is stored (called the Host Computer), launch fileMaker and opoen the file. If you then go to one of the other computers (called a client computer) with FileMaker installed, can you launch fileMaker, Select Open Remote... from the file menu or by clicking a button labeled "remote" and see the name of the first computer in the dialog that pops up. If you click on that computer's name, do you see your file's name appear?
On the Host computer, use Filemaker | Sharing | FileMaker Network.... to turn sharing on. Then select your file (It must be open when you do this), and specify "All Users".
FileMaker checks license keys and refuses to open a hosted file if host and client have the same license key. I don't know how or if this will work with the demo version due that anti-piracy feature if both host and client computers have the demo version installed.