Unlike Google Input Tools (Unicode), Dass Anand uses a specific keyboard mapping. If you just install the font and start typing in English, you will see English letters, not Tamil.
You need a Tamil ASCII (TAB) keyboard layout. Options include: dass anand tamil font better download link
Instead of downloading a single risky file, it is safer to download a "Tamil Legacy Font Pack" which usually contains hundreds of fonts including the one you are looking for. Unlike Google Input Tools (Unicode), Dass Anand uses
On Mac, you can install the .ttf file via Font Book, but you still need a TAB keyboard layout. On Linux, use font-manager and an old SCIM input method for TAB. Not recommended. On Mac, you can install the