Download Malithi web font for Mac from here (.ttf).But still with some rendering problems may be So I just thought of posting this few simple steps on how to have Sinhala unicode running for a Mac.
Then through wikipedia article on Sinhala Font Guide, I was able to get a link to download Malithi web sinhala unicode font designed for Mac. For OS X (Leopard, Snow Leopard) site did not contain any information. When I initially checked out about this I could not find this details from Siyabas site by ICTA where it has all other information regarding installing Unicode fonts for different Operating Systems. I was looking for how to install sinhala unicode fonts for Mac.