Installing the font is only half the battle. Since MCL Kannamai uses a legacy encoding, you cannot type in Tamil with it using a standard English keyboard. You will need a that maps to the TSCII or the specific encoding used by the font. Here are a few approaches:
Based on online discussions, the "repack" versions of MCL Kannamai often feature: mcl kannamai tamil font repack
Drag and drop the extracted MCL Kannamai .ttf or .otf files into the Font Book window. Common Troubleshooting Steps Installing the font is only half the battle