Q: I have home sharing setup on all computers, but can't see shared libraries on one computer

I have all computers on the same network, and have setup home sharing on all of them (two PC, one Mac). I have a "Home Sharing" button on the 2 PCs and can see the two shared libraries, but on my Mac I have no "Home Sharing" button and can't see the libraries. All my iTunes software is updated to the latest versions. Does anyone have any ideas to help?

Lisa