i bought the game for Windows Pc, before a time.
Now i use Mac. Its it possible to play it on mac or i must buy it again for Mac ?
If you bought it direct from Telltale, both the PC and Mac versions should work for you. Click on "MY ACCOUNT" there in the upper right corner, then "My Games", then "Tales of Monkey Island", then change the dropdowns to Mac before downloading.
If you bought it from any other source, then I don't know. You can send mail to firstname.lastname@example.org though, and see what they'll do for you.