Might sound obvious, but are you sure the share name is Multimedia? Where you enabled sharing it will tell you.
Also, make sure the computers can talk to each other. On the server one go to start, run type cmd and press enter. Type ipconfig and get the IP of that PC. Then go to the other PC, open CMD again, and type ping (ip address of server). If you get a reply then they are talking, if not, there is something wrong with your network!